i use a sospenders belt pac. it doubles as a wader belt and is only 3x4x9. it's got a lanyard to manually fire it as an automatic would be useless in the surf. there is a tradeoff with it as far as safety goes, not only do you have to pull the lanyard but you then have to duck your head down and slip it around your neck for it to work properly both of which require you to be concious. a bad fall in the rocks is a risk.
i figure i increase my odds by wearing it