What does “payable at base” mean?

If the Boat Owner requested the refundable security deposit to be payable at base that means that the Renter will need to pay it directly to the Boat Owner, on the day of the trip, before embarking on the boat. For more details about the form of payment (cash, credit card, etc), the Renter should message the Boat Owner directly using the “Message Owner” button on the boat page.