yep u can overzinc.
boat: mostly benign
bonded- alkali bottom paint
attack around thru hulls. kind of dissolves paint
leaving a rough moonscape for a few inches in the paint
around metal items.
unbounded-not much of an issue unless your prop is painted. watch rubber components like cutlass bearing which are in close proximity and may not like alkali.
on wood boat: depends
bonded: alkali ears soft wood and destroys hull
integrity around thru hulls. exhibits white feathery spider webs on surface of wood. very bad.
unbounded: same as fiberglass
above. remember however wet wood often creates an accidental bonding system so be careful.
over zinced is as bad as overzinced. u live in a battery
. get a good silver half cell and a millicolt meter and keep your voltage in the 800 to 900 mv range. certainly below 1100 mv.
good luck. I carry a home built corrosion
meter and measure regularly.