Go to a SCUBA shop. There are several products you can use to store valuables and take them with you while diving. Some are as simple as a plastic bag with a lanyard
http://www.leisurepro.com/Prod/Categ...344.html?Hit=1, to an Otter Box
http://www.leisurepro.com/Prod/CategoryID_970/Context_965/Sort_Stock/DescSort_0/OTR2.html?Hit=1
I've used both to protect rental car keys, hotel keys and money while shore diving. As long as you have a pocket, can tuck it into your wetsuit or have a place to attach a lanyard, you should be golden.