Summary: While you are fully submerged in water many of the item actions on thrown explosives do not work as expected.
Game Version: A21 b324
Game mode: Client on Dedi, RWG
Did you start a new game? Yes
Did you validate your files? Yes
While you are fully submerged in water:
Thrown rocks/snowballs: work
Molotov: no interaction possible
Pipe bomb: Can light. Can not throw. Explodes in your hand.
Stick of dynamite: Can light. Can not throw. Explodes in your hand.
Contact grenade: Can pull pin. Can not throw. Does not explode.
Timed charge: Can arm. Can not throw. Does not explode.
I know that at least in some cases (maybe all) this is in XML, such as for the pipe bomb
<requirement name="CVarCompare" cvar="_underwater" operation="LT" value=".98"/>
I can see that none of that is consistent but did not test or investigate any further.
Expected result: Some issues are obvious like not being able to light a pipe bomb underwater but whether they can be thrown unlit or how other types of explosives should behave is more of a design issue.