You probably should be using magnesium anodes in freshwater.

Magnesium is less reactive than zinc; you are interconnected to ALL
OTHER BOATS in your marina since you (not such a good idea) keep the
AC connection at all times when you're in your slip. The problem will
be that if you are essentially bonded (via the AC ground) to another
boat(s) that does have magnesium, YOUR zinc will be the one that is
consumed since it is the least noble.
Consider to install a 'galvanic isolator' so that your boat is
'isolated' from that common AC ground wire !!!!!!!!!! ...... then get
a calibrated electrolysis cell (corrosion reference cell) and be sure
that you have the exact AMOUNT of anode needed for ***your specific
location*** for the best or most effective protection ........
http://www.boatzincs.com/corrosion-r...electrode.html. .... a
lot of good info is found on the www.boatzinc.com website.