I have flown in both dingy dry suits and proper air crew ones. Try Survival 1 in Aberdeen for the made to measure suits, they have lots of pockets on them. Always wear the suit (you will not be able to put it on in the event of) and dress for the season, or more importantly the sea temperature underneath the suit. There are plenty of life jackets around from the basic pouch on your waist type but IMHO if you are serious about any prolonged over water flights then equip yourself with a life jacket that has a hood on it. If you can cover you head you increase your survival time considerably.
A cheaper option could be a basic life jacket and a wetsuit hood and gloves.
Get a PLB also then the helicopter has something to aim at and if you have any space or money left then buy a raft as well. If however your over water legs are likely to be no more than 10-20 minutes then this might be a bit over the top as it would be simpler to climb to 5000+ and minimize your time out of gliding distance from land.