hi surfwatcher,
no offence my friend, but if you look for a cheap boat ride you can catch up with the boys right on the beach in the whole puerto plata or boca chica/santo domingo are right when you are there, there are small speed boats for rent everywhere.
if you want to rent a boat on the east coast, you mentioned punta cana as a area, you should do that only if you want to go for big game fishing, during summer time we only target Billfish like your mentioned Marlin, of course we get dorados wahoo bonitos aso all over the year here, but not in the quantity to target them on daily bases during summer season, that's the time we are one of the worlds first class spots for the big ones, and of course a reliable charter on that would be a 500+ for a half day.
enjoy our beautiful island
Mike