Ho Yay: Between David and Mark. You'd think there was something there that David didn't want Kay to know about...
Then again, maybe not. Notice that Mark glares at David when he gets close with Kay, and shows off his new spear gun to both of them.
I Am Not Shazam: The monster is never actually referred to as "Creature from the Black Lagoon". He's called "the creature" a few times, but never the full title. His official name, especially in the second movie, is "Gill Man".
Nightmare Retardant: Posters for the film developed an odd tendency to depict Gill Man with a pair of clownish red lips.
Strawman Has a Point: Mark may be Jerkass, but if they'd listened to him, they'd have wound up a lot better: If they'd killed the monster (who they knew had killed at least four people previously, at least 2 without provocation) they'd have been fine. If they'd have left when he wanted to, rather than taking pictures of the cave than the monster probably wouldn't have broken out. And if he hadn't gone underwater after David, than David would have almost certainly died (although this last one was done out of purely ambitious reasons and no actual caution).
While the bonus multipliers are displayed as 2X, 4X, 6X, 8X, and 10X, in reality they are 2X, 4X, 8X, 16X, and 32X respectively. Custom ROMs fix this, but also make the bonus far more substantial despite the lower multipliers.
The Snack Attack hurry-up underflow glitch can lead to scores near the Cap of 9,999,999,990. The hurry-up will sometimes wrap around to a value very close to that cap. A player with a low enough score can intentionally let the hurry-up run long enough so that collecting it will not make his score exceed that cap, and have a nigh-unbeatable high score.