Once you enter your fleet into the system and if your fleet is all based in the same base, the system can offer only that base as a start and end point of any booking since it is the only base that has boats based in it.
If you want to have more bases in the system to be more flexible in your bookings, for example if you want to use one-way bookings, you will have to base boats in bases that you want to use.
If you do not have real boats in other bases you can use boats that only serve as placeholders for them.
So enter a new boat, this boat can be of any model and it does not have to have prices or equipment or anything since it's purpose is to serve as a place holder for a base.
Name the boat appropriately (1.) to distinguish it from the actual fleet on first sight and, as it's base, select the base you wish to have available in the system.
What is important is to place checks on Disable booking (3.), Disable on Web (4.) and Disable yacht distribution (4.) since this boat is not supposed to be visible anywhere.
Press Apply (6.) when you are done.
Once you set up the placeholder boat, the system will let you select the base for which you entered a placeholder boat in Reservations in From base and To base dropdowns.