The ole Switch-a-roo. In this gamemode, you have to stay underwater instead of above it! All the props will remain the same. You will need to build a boat which allows you to attach to and stay underwater. A heavy metal would be perfect. Floatable props can also be useful as it can act as a last resort as a roof, to stop you from floating above the water. After making your boat, you would attach yourself onto one of these props so that you remain underwater. Just be aware that you can only attach yourself onto 2 of your props, so choose your props wisely. When the round starts, a balloon would magically attach onto your back. This balloon will not take any damage or affect the movement / aim of the player. When the player is exposed to the air, the player will start taking damage, The player could possibly use the harpoon to get back down, but there will be a time limit that the player will be able to use the harpoon for.
This would be an interesting idea except for the fact that none of the guns can fire underwater. Perhaps an alternative to the current weapons we have can be found?
Well, I didn't consider that now did I.... Yeah, you could probably avoid this problem by adding a new specific gun only for this particular event.
Only having one gun seems boring but I don't know for sure. Might have to find something on the workshop.