If you are not fussy with quality of photo, then ISO 200 to 400 film speed should do it.
If not fussy, use film not slides. Film (negative) is generally more forgiving.
Since you are snorkeling you will be on the surface where there is lots of light anyway.
What brand? Alamak!!!! Kodak, Fuji, Konica... what ever is mentioned will not go down well with someone for sure. My own bias is towards Kodak, but I also do use Fuji now and again.