Whether dogs are good swimmers or not depends on their breed - although all breeds CAN swim, some are better than others.
The problems with dogs swimming is that their back end must float... if that starts to sink they will get pulled under and drown. For this reason, I'd recommend a lifejacket - and also the fact that it makes it easier to lift in a wet dog.
Get him used to the lifejacket before you go boating. Just let him wear it around the house for a short while until he's happy wearing it and doesn't try to get out of it.
The various groups that train Newfoundlands as Water Rescue Dogs recommend a large piece of VetBed - http://www.petlifeonline.co.uk/Store...etbed-Original
- be put over the tubes when the dogs are getting in or out. Firstly, it protects your tubes from getting scratched, and secondly it gives the dog some grip to jump. It might also be good for him to lie on - as it'll stop him skidding around on the deck.
If he is as nervous as you make out, perhaps make a few trips out to the boat, get in it, switch the engine on and don't go anywhere. Just to let him get used to the noise and smells before he has to deal with any movement.
Remember how sensitive dogs noses are - that outboard is really going to stink to him.
Perhaps take him for a walk, put his lifejacket on, take him to the boat and start it up - making a really big fuss of him, lots of treats and praise for being chilled out about it, and ignoring him if he gets stressed out by it.
Start off with short trips on calm days and work up from there.
Hope that helps,