The Mariner basically fails pretty hard to emote at all throughout most of the movie (blame Costner). On the other hand...maybe it has something to do with him being part fish. Think about it.
Much simpler explanation for the understated acting job: the character spends something like 95-99% of his time with only himself for company, with brief interactions with (likely equally undersocialized or possibly crazy) sailors or suspicious, backwards atoll-dwellers (who he seems well aware would kill him if they knew what he was) being his only interaction when he does communicate with others. He's probably lived that way his entire adult life, why exactly would he be an emotive and charismatic sort?
This troper just watched the movie for the first time in a few years and realised an extra dimension to the mariner flipping out at the beginning when his limes get stolen - he needs them for the vitamin C.
The Mariner's fascination when he sees the gull perched on the balloon's gunwales isn't just because it's a kind of animal he's never seen before. It's that it's the first living creature besides a person which he's ever seen that can stand on legs, recalling Helena's earlier remarks that humans were built to run, not swim. Even if he hadn't spotted the mountaintop just afterwards, the seagull's basic anatomy confirms that dry land still exists.
The Mariner has an apparent knack for fish eyes, which is mostly played for laughs. But numerous cast-aways that had to spend weeks or even months on sea and only could fish for food developed over time similar taste - fish eyes are the only part that have different taste and when everything you eat tastes the same, any variety is appreciated. Bonus point for being (relatively) sweet when compared to the rest of the meat.
Moreover, the eyes would contain different minerals from the flesh of the animal, and the fluid inside the eyeball is a source of water.
If there's no one living on Dry Land, how could a map to there exist?
They do make a reference to longitude and latitude, but longitude is impossible to determine on a uniform, featureless sphere, so the point is still valid.
There were two dead bodies on the island. They could have tattooed the map on their daughter and sent her out to sea to bring others. Of course, there isn't much to go on in the movie for that theory but it makes sense.
The highest point of Dryland has a geological survey marker identifying it as Mount Everest. If they're sufficiently adept with Lost Technology to know how to use coordinates they're clever enough to know where Mount Everest is.
Here's another headscratcher: Assuming that these Atolls are all built using salvaged materials from the ocean floor, what's keeping it all afloat?
The shape? A rock, properly shaped, can float, despite it sinking, well, like a rock in normal circumstances.
Indeed, it's called a metal-hulled boat.
Some might be built on chunks of tufa, which floats in water.
How did the beach erode away to sand in such a short amount of time geologically?
Well, to be fair, if the oceans rose that much, the atmosphere would rise with them. And Nepal's latitude is similar to Florida's, so it's not that far-fetched that it would be tropical.
Forget that, how are there horses on Mt. Everest but no sign of human habitation save a few mud-and-stick huts and some long-dead corpses?
Possibly some of the last survivors of the original sea-level rise had to ride horses to get to Everest in the first place. Their descendants died out, but their horses' descendants didn't.
People live in Nepal today. Why wouldn't they have clung to that land?
Beach sand is mostly ground-up coral that was gnawed off reefs and pooped out by parrotfish and other coral-pickers. It just washed up there, same as it does on beaches today.
How the heck could there have even been enough water in the first place to swamp almost the entire surface of the Earth? The idea that even Antarctic meltwater could cover, say, all of Colorado is preposterous.
How is the dirt the Mariner digs up even useful? It has been sitting under salt water for who knows how long. This soil would have such high salt content, nothing edible could be grown from it.
Presumably, they've found a way to purge the salt from the dirt. Helen even mentions that the dirt that was packed in Enola's baby basket was far richer and darker than the stuff the Mariner traded.
Flush it with clean water. Sure, that's still a valuable commodity, but much more common than the dirt. Or just mix it with the compost they produce in quantity. Alternatively, they don't know nothing will grow on it.
Just collect it in a bag of fine-weave fabric, hang it from the mast, and let it get rained on. It'll take many months if the weather isn't cooperating, but eventually the salt will get washed out of it.
As the Mariner and Helena escape, he shoots a harpoon at the gunship and tows it to blow up the Big Bad's boat. Except, there were two kids on that boat who were using semaphore flags. It's not shown, but only the Big Bad manages to (just barely) make it off, and he loses an eye in the process.
Signalmen, be they drummers/pipers of antiquity to the radiomen of modern day, have always been legitimate and highly sought out targets on the battlefield. They may be children, but they are also directing an outright slaughter of the people of the atoll - men, women, and children alike. Why should they be spared?
He also blows up the Big Bad's boat at the end for no particular reason, which is packed with dozens if not hundreds of men and women who simply want to find dry land, taking out what could well have been one of humanity's last hopes to establish a stable, genetically-diverse population.
Not so much genetically-diverse, as the Big Bad is apparently cousins with most of the Smokers. They're already significantly inbred and a few more generations wouldn't help.
"Cousin" may simply be a term of endearment, the Deacon saying that they are all family.
The joke is that it should just be that.
Actually it is stated in the novel that the feral Smokers are the textbook example of Humans Are Bastards such that even the small young children are amoral sociopaths.
Since we are already on the subject of genetics - it requires a population of about ten thousands to avoid problems with inbreeding. Another five to be extra sure. Not even Smokers had such numbers, so humanity is screwed anyway.
Possibly if the Mariner encounters others who pass muster as decent people, he can send some of them to join the group on Everest. Maybe not enough for the population to thrive, but at least enough to get them through the current bottleneck. After that, inbreeding may well be a good thing, if it brings out more gilled mutants who won't need Dry Land to survive.
There is no such thing as "good" inbreeding, at least not when it comes to more complex species, not to mention humans. One of the thing inbreeding hits hard are mental capabilities.
It's alluded to, if not flat out referenced, when the leader of the atoll looks to the man with the thick glasses and says that looking to their own too much for impregnation is "undesirable".
Species have survived and come back from far smaller populations than ten thousand.
Yes, simple species have come back from less than 10,000 individual members when closely monitored and maximum genetic drift is ensured by forcefully introducing mating pairs from different areas. Humans are much more complex, and have many more opportunities for something to get genetically wonky. Unless there are atolls with populations of thousands (which didn't seem to be evident), humanity is on the way to extinction. Downer endings galore.
Wasn't it always the whole point of the entire story that humanity is on its way out one way or another, but at least the main characters reached the dry land and can now live peaceful lives?
Given the Elder's reaction, the idea that the Mariner refusing to have sex with a girl must mean that he's a Smoker spy and not, for example, gay or concerned about whether she actually does consent to having sex with a complete stranger really throws up some disturbing implications about the methods in which this small community has employed in order to survive.