the are a couple of things i think nos's guide doesn't mention:
- fire extinguishers - search here there are various threads
- torch / lights. even if you don't plan to be out at night - make sure you have some sort of half decent torch (with spare batteries - that reminds me I must replace mine) so if you do breakdown you have a chance or attracting attention after dark
- when you go afloat - tell someone ashore. make sure they know where you are going and when you are due back. I usually say - i am due back about the time I expect to get back and then tell them to call the CG if they have not heard from me within an hour of that time. Make sure they know how to contact the CG - and have details of your boat (see cg66 below) and where you were going etc. If you do get delayed try to contact the person ashore with a revised eta - if not able to then tell CG.
- register your boat throught the CG voluntary cg66 scheme - it costs nothing but makes their life easier if they have to start a search. see www.mcga.gov.uk for details
- take spare waterproof clothes with you in a locker or dry bag. as well as potentially saving your life in the event of an emergency or man overboard needing dried out - it is amazing how much the wind chill is at 20 knots and this can keep a crew member warm preventing it being a miserable day - or you having to turn back sooner than you would like.
finally if you have not already done so - consider some training, not only will it make you a safer boater, but it should help your confidence and improve the experience for you and your passengers.
PS most RNLI callouts to powerboats are engine related (a lot of them just out of fuel) - so if you decide not to go down the spare engine route (I suspect with a smallish boat like yours, like me, you will struggle for space) it would pay to learn a little about how your engine works, the most likely problems and how to fix 'em at sea (and that means you need a few basic tools - including a sharp knife to cut the prop free if it gets fouled on pot ropes etc).
PPS extra skill / care required for pulling toys/skiers etc. Beyond my area of expertise.