I've used both heavy, industrial gas washers and cheap electric Karchers and everything in between and for working on my own boat I'd go with a cheap electric. It's easier to hump to the boatyard and doesn't run out of gas in the middle of the job.
The job IS longer though but they do just as good a job when it's done. They are cheap and last for several years - you can easily go through 3 or 4 before you match the cost of a bottom end gas unit. I've acquired both of mine with Air Miles so they were free.
Be very careful of anything soft like the bellows on outdrives (don't ask!) or around speedo paddles etc. Use the rotary nozzle on the bottom but NEVER take it on deck or use it on the topsides - they are extremely intense and I'm sure you can do a lot of damage very quickly if they get too close to the surface.
Stick to the fan nozzles for those surfaces. I agree about teak but if it's going to need sanding anyway, go ahead - you'll be amazed at how much salt comes out of unfinished teak! It was like I was swimming in the ocean the whole time.
Finally, my washer is just about my "most used" power tool - way beyond even a hand drill - cars, driveways, sidewalks, boats, cedar shingles, house siding, etc. etc.