Tom, if you want to stay on the Merrimack, check over in Salisbury, there might be slips available over there, and cheaper.
Salem has a couple of options. I use the Salem Water taxi, your rental of the mooring includes launch service. Simplest way to get on the water in Salem. Note, I've only been doing it for two years there. Interestingly, the first week we were there we met two couples, from Newburyport, who had just moved there boats down to Salem because they were tired dealing with the Merrimack and the power boats. The taxi is managed by Hawthorne cove Marina, which is part of the Brewer Yards.
There are several private clubs in the area, Salem Willows, Palmer Cove and Jubilee. Long term they can be cheaper than the taxi, but there is the high initial cost of joining and buying a mooring. Also, some have a waiting list. The rest of my family is involved with other things so I haven't pursued joining a club.
Send me an email through my profile if you want more detail on the clubs.