You can use the Authorization and/or Release by Tenant form (BUS-23) [checkbox #5] in the Members Only part of the TSSA website for this purpose, or simply draw up your own simple agreement that states the tenant is abandoning the boat and trailer and gives up all future claims. The form is ready-made for this type of use, or for abandoning a regular storage unit. It does not however, say anything about forgiving the debt so if that is important to this tenant, you may want to add that.
As Jeanne stated, it is completely a business decision. Just be sure you get the tenant's authorization of abandonment to you in writing. If you are planning to sell the boat and trailer or re-register it, getting the title transferred and the boat re-registered may prove to be something of a challenge. For that, you may want to explore the forms the TDMV and TPWD have specifically for that purpose. There may be something the tenant should sign for both the trailer and the boat that will make transferring title and re-registering a good deal easier. Generally even if someone gives you something, the state will want to collect taxes from the buyer based on a reasonable value of the property.
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