It's been awhile since I have been over there but you should be about 3 1/2 miles from the bridge at Rt 1. Leave a half hour or so before slack tide and you should be fine as long as you can chug or sail along at 5k or so. Check your tides and current forces in some publication like Eldridge and have a fantastic wonderful trip. You should not have too far to go to clear the Merrimack hence not a big issue if you time your departure prudently.
It's better to have your enemies inside your tent pissing out than outside your tent pissing in...