The Spyderco is an excellent suggestion, if he likes a folder. When I snorkel I always prefer a fixed blade. My favorite for scuba diving is definitely my Buck 187 Intrepid chisel point. A lot of people say these don't have good enough rust/corrosion protection, but I just clean and dry my knife after a dive, and have never had so much as a spot. These aren't made anymore, but you can usually pick them up on eBay for around 100 bucks. For snorkeling, my Intrepid is just to big, and so you will probably wanna use something smaller. I use a Buck 185 Triburon, but they are very hard to get, and more than 100 dollars. I also love to use my Buck/Mayo Kaala, but some say its to small. And it really is little, at under 7 inches. So a couple suggestions would be a Kershaw 1006K for a inexpensive but decent knife. It is 420 J2 steel though, which may be under par for corrosion depending on what kinda stuff your cousin does. Otherwise a CRKT CR2604 is a nice little knife, with AUS 8 steel. These are both in the 40-50 dollar range. Of course neither of these are all that great, but I have friends with them,and they perform really nice for the money. Another buddy has a Benchmade 110H2O, and I do love that knife. It runs for around 100 if I remember right, and is super nice. Hope this helps some.