If your neighbor is using the lot with your permission, his occupancy is "permissive" not "adverse." Don't worry about it.
Disclaimer: This answer is provided for informational purposes only, does not constitute legal advice, and does not create an attorney-client relationship. Actual legal advice can only be provided after completing a comprehensive consultation in which all of the relevant facts are discussed and reviewed.
Permissive use is not adverse possession. However, I suggest you put something in writing to indicate that you granted him a license to park his boats on your lot and such license is revocable at your will. Your neighbor could claim that he is your tenant and has a leasehold interest in your lot. If he claims he is a tenant and he does not remove the boats when you want, you may have to bring an eviction action.