These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Bomber enemies drop a stream of bombs almost constantly. If the bombs are damaged, they just turn into flaming bombs that can still destroy your submarine.
The Antarctic Ocean Stage as well as the final stage has the golden sea mines. These explode into a burst of eight indestructible bombs when destroyed. They also can start a chain reaction with other sea mines, causing a lot of smaller bombs to be fired off...
The Deep Dark Sea stage has grey, futuristic deep-sea submarines. That fire off indestructible aimed shots. And start coming in a column of SIX. And they come at you indefinitely unless you progress further to the right... The only good thing is that they can't take much damage. Starts at 2:25 here.
Almost all the stages have annoying little yellow robots that drop bombs on you if they are above the water, and launch homing missiles if they are underwater. They also come in great numbers, and sometimes they will just drop on you from nowhere.
Annoying patrol boat enemies that spawn endlessly will try to drop depth charges on you in some areas.
There are also endlessly spawning helicopters that do the same too.
The Deep Dark Sea stage. At one point you have to manoeuvre through lava vents that spew out lava balls that travel in annoying arcs. At another point, you are pitted up against endless waves of high-tech submarines that fire INDESTRUCTIBLE aimed plasma shots. The first few waves only consist of two or so, but later on you have to fight against SIX of them in a column from the top to the bottom of the screen! Not to mention that the three-headed dragon-whelk J3 was That One Boss too. Prepare to die A LOT on this level...
The first is J3, a three-headed, fire-breathing cross between a dragon and a whelk. While its attacks are not too hard to avoid, there are lava vents around the room that periodically spew out fast-moving fireballs that can easily kill you if you're not careful.