The easiest place to park is usually in the city lot behind CVS across the street from FLOAT, where there are 3-hour meters that accept credit cards. Parking is also available at 2-hour meters on Medford St and along Broadway. Meters are $1/hour, enforced Mon–Sat 8am to 8pm.
Free parking is available around Trum Field on Broadway, a couple blocks walk away from FLOAT, on Medford St on the Medford (north) side of Broadway, and on Winter Hill and Albion streets.
If you are at a meter, we suggest you pay for half an hour longer than your appointment time, to allow for showering and ensure you don't have to sprint out of the center the moment you're done floating. Let yourself enjoy the slow-moving floaty feeling you'll probably have when you get out of the tank.
For anyone arriving at 7am, Monday to Saturday, there's a complication. The meters behind CVS are "smart" so they won't accept payment outside of the hours when payment is required. That's a problem for anyone floating from 7–8:30 am, because your meter will start being enforced at 8:00 am and there is no way for you to pay for it. We have pointed this problem out to the City, but they don't have a good answer. (Unfortunately, we typically only have one staff person in the center, so we can't run out to feed your meter.)
So, if you're arriving at 7am, Monday to Saturday, we suggest your best plan is to park in the free spaces on Medford St in Medford (across Broadway from FLOAT), to avoid the problem. You're here to relax, and the last thing we want is for you to have to stress about parking meters.
(On Sundays and holidays, meters are not enforced at all, so no need to worry.)