At some point after you fix Dagny's heater --
How do I fix Dagny's heater?
-- someone will ask you to go out to the lighthouse to fix the light. The only way to get there is via the dinghy.
How and when do I use the dinghy?
The dinghy only goes to the lighthouse and back, so you don't have to steer; just get in and go. If you get there and you can't get out, the tide is too low. You'll need to wait for high tide to come back.
How do I change the tide?