Zinc bolt replacement on 2004 formula 41PC transom

[Stanley pool]

Diver says he can’t get nut off bolt to replace rectangular starbourd zinc .. says it’s “spinning”
Can this be done in the water( sea)?
Will it be grounded if replaced?
Is there at kit for the bolt and nut?
Can’t get anywhere near the transom from inside the engine compartment.
Photo below shows nut unscrewed to the end of bolt.
Thanks to anyone who can help
Stanley

 

[alldodge]

Pic doesn't show up, but a diver that does this kind of work for a living should have seen this before. Ask around other dive shops and see what they say.