I'm a surfer and I hide my key all the time when I'm in the water. It doesn't matter where. You don't have to hide it very good. Theives don't look for keys. They will just bust in your window. Happens all the time up here. Seriously.
Those magnetic boxes will fall off. If you lock yourself out regularly keep a copy in your wallet. I also have a key in my surf racks. Again, believe me. No one will look for your ding dong key!

85 4Runner 22re, mostly stock. Over 340K miles(SOLD for $4000 7/06)

98 2.7 4cyl 4Runner Bone Stock perfect 200,000 miles (alright, it has a HG and clutch in it's past)