Underwater walking

Usually this is because the character is too heavy to actually swim or dive (for example, due to wearing a diving suit), or the game doesn't even bother trying to simulate either.

Besides modern games where the actual reasons are likely covered (not necessarily though, and some older games may have them covered as well), in older games this was mostly a matter of course due to limited simulation of various environments, causing water to behave simply like air but with greater (air) resistance and reduced gravity (character moves as if in slow motion but jumps the same or even greater than before).