ISO Standards are 150N and 275 N, I wear a 275 N Lifejacket and my surface dry suit...
on the sea survival course I was the only personal on the course that could not drown then self. All 150 N lifejacket the person could temp turn them self over on face, some were 175N.
Some life jackets state a bigger size but rely on you to provide the extra top via The tube!
Yes - they are PitA to get in bost, hence after sea survival course I bought dry suit, and fitted narrow ladder to back of the boat to aid entry.
One trick if you fall overboard is if you can deflate your tube of the RIB to get back in.
lifejacket videos can be found on sailing club website;
these are self explanatory, also the cold shock boot camp.
I asked at my Lifejacket training course about 190N jackets and Drysuits and I was told recommend 275N.
I did my Sea Survival Course with SeaSkills.
Optional Extra's ; Crutch Straps, Lights and Stray Hoods are more essential if you want to survival in the water.
The think I do, the best lifejacket on the market is the one that you wear, don't buy anything without trying it on to see if it is comfortable...