Yes, you may deduct the cost of the fuel used to power the rented boats and jet skis. That is certainly part of the cost of doing business.
Here is a link to Publication 535, Business Expenses. You may find it useful in your business
https://www.irs.gov/pub/irs-pdf/p535.pdf
https://www.irs.gov/site-index-search?search=Business+expenses&field_pup_historical_1=1&field_pup_hi...
**Disclaimer: Effort has been made to offer correct information; but due to the discussion forum limitations, the poster disclaims any legal responsibility for the accuracy of the poster's response**