>>531690871
>but I don't remember there being any downside to being in one.
Most Nikkes that one use don't actually see combat themselves. Aegis Squad is just for show, and even if they did see combat they would be using mounted artillery instead of themselves.
The swimming body is made of less-dense material, and probably breaks very easily compared to a regular Nikke's denser body. Even got brought up in the Stellar Blade event.