They tend to be so slow and that isn't really fun. Uncharted 4's underwater scene was only ok because of how unexpected it was imo. Underwater platformers are also slow. HBU?
Mario kart 8 is the only game I can think of which has great underwater levels. Most games they're just crud.
I have mixed feelings.
Astrobot's was great.
Depends on how well they are done. If done well yeah they are fun