In answer to the original question....
I'd go with Handheld like others...good coms is the foundation of safe Boating
A good loud whistle is good and really cheap!...and I'm not talking about the cheap lightweight plastic things!...A good old "Acme thunderer" is a good "starting" point and can be bought for a few pounds...and comes in handy for everyday hailing people on shore and other craft/water users with no coms.
Apart from the usual couple of good Torches..I always Carry a few Glow sticks in various colours.
I was anchoring overnight at one of my fav camping spots some years ago and lost my Nav lights...a couple of red and green light sticks taped to the A-frame made me sleep a lot better while the RIB was at anchor 200m away In the Bay.
Ofcourse carefully checking and inspecting (and Maintaining!) your Craft..Kit and..Trailer on regularly is probably the best tip of all....and completely free!...The checking Bit anyway!