just thought i'd ask if certain bow or missile wapons would work underwater in this case im thinking of the Modified harpoon gun since it is created by a race that are amphibious?

I guess it just comes down to common sense.

For obvious reasons, you couldn't use a sling or bow underwater. If there are harpoons in-game, or spearguns or whatever, I'm sure they could be used in such a fashion.

If you're unsure about a particular weapon, ask a DM in-game to get a certain answer.