Magnesium anodes are more galvanically active than zinc anodes and provide superior protection for metal components in freshwater, which is less conductive than saltwater.
Magnesium anodes will become very active in saltwater where they will probably only last a matter of months.
If you berth your boat in freshwater, but use it in saltwater for more than seven days (or 14 days in any one year), you should consider changing to aluminum anodes which perform well in a broad range of electrolytic environments.

This anode can be welded on, or bolted on using the 5/8" holes in the strap
Strap dimensions are 1-1/8" wide x 1/4" thick x 14-1/4" long tip to tip
Strap has 5/8" holes on 13" centers