Can someone tell me if there is a ferry service from Hua Hin to Rayong or the best way of getting there? Thanks

Helicopter is probably the best way, particularly the ones with hot flight attendants...

If you're thinking of the one that ran to Pattaya for a while it went bust.

hire a jet ski

Cheap and pleasant? Train to BKK then BTS to Ekkamai.

Cheap and faster? Bus all the way, ideally middle of the night to avoid the worst of the traffic.

Fastest but not cheap - taxi/van/private car, probably find one for 3-4K if you ask around, plus tolls of course. Or offer 3K and you cover the petrol. Might have to ask twenty before you get a good response, maybe increment by 500 each 10 no's you get.