Re: Marine handheld small enough to fit in life jacket?
Kokatat (among others) makes kayaking PFDs that will nicely carry a handheld radio. The pocket in the center of this one is designed to fit most handheld radios:
Kokatat | OutFIT Tour PFD - Sea Kayaking - Activity
These are cut for kayaking, but as class III PFDs they work fine on sailboats too. Most of my "good" PFDs are kayaking ones because I was into sea kayak touring before I got into sailing.
boats that I sail on regularly: Pearson 28-2, Rondar 505, Yankee 30, Blanchard Junior