Robes' Oregon 450 is waterproof, but I bet it will sink like a rock if it goes overboard (I've got one.) I was comfortable parking mine in a drink holder, but TakeFive suggested a handlebar clamp that Garmin offers if you are concerned about losing it. If your charter boat has one of these open transoms, you might consider a clamp or tether as cheap insurance.
The handlebar mount can be found for $8-10 and is cheap insurance that puts the handheld in clear view. A drink holder might work like Frank mentioned, but you just won't know that until you get down there.
Believe it or not, my Oregon became my primary nav device at the helm during my BVI charter. The boat had a big Raymarine chartplotter, but I had no way to transfer routes or waypoints into it from my computer. I turned the boat's chartplotter on and set it to "course up" mode and keep the "own ship" icon in the middle of the screen, and from that point on I merely used it to get a broad view of the area. Any critical decisions about where to change course or cut between islands was done by following the purple line on the Oregon (using routes that I had pre-planned on my computer using OpenCPN and the NGA charts that I had calibrated for use with OpenCPN).
Based on Robes' comment about HomePort, I looked up some detailed specs for the software, as well as user support messages, and it looks like even though the Oregon is not listed on Garmin's webpage as a compatible device, a recent software update allows HomePort to pull the preloaded charts out of my Oregon 400c
. Since I therefore already have the charts to make it work, I sprung the $27 for the software from Amazon. I'm looking forward to trying it out.