FMJ would be the armor piercing one, HP would be the soft target killer, and flechette/dart is... not going to do much better in water than others.
Options for less conventional ammo would also be interesting.
Like for explosive weapons.
-High explosive ammo.
Increases AOE, decreases damage.
-Fragmentation type.
Slight delay on explosion, lower AOE blast damage, but fires out a series of projectiles in all directions that can potentially hit stuff at triple the normal AOE range.
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Incendiary. Produces 30% less initial damage on detonation, but deals 40% of that initial damage over five seconds (better against big enemies, less useful against smaller ones.
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Shaped charge. Cuts the AOE to the far sides hemisphere only, but it now reaches out the distance from that point of it's original diameter and deals 20% more damage. Your misses are less forgiving and you can't bombard groups of enemies from above as easily, but with more ground level fire you can get a group of enemies good, and for larger types it does more damage.
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Concussive warhead. It's less flashy and bright, and deals 40% less dmg. But it can send small enemies ragdolling through the air. So it's sort of a stun weapon, but also just hilarious to use from time to time even if not the most practical for serious fights.