Nice fish and corals.
Perpetually moving fish can be challenging subjects.
F3 does not provide much depth of field (amount of subject in focus around the focal plane). For stationary subjects, like corals, if you could put the camera on a tripod, you could achieve greater depth of field by using much longer shutterspeeds and a smaller aperture (higher f number). I often exposure for say 1.5 seconds at f13.
If you aren't comfortable shooting totally manually, try Av mode (aperture priority). Set your fstop and let the camera determine how long the shutterspeed should be to work with that aperture in your specific lighting situation.