If you want to take a boat out to sea in Bitlife, you must first purchase one and then click on it under the asset tab. Under this tab, you can find your boat listed among other items owned by you, click on it and then find the option ride.
This option will allow you to take a trip with your boat out to sea, but be warned that you might have a dangerous encounter if you do it too many times. In short, this is an accident, and if you don't know how to swim, there is a high chance you might die.
That said, you can learn how to swim by joining the swim team while you are in middle school. To do this, simply hit the school activities tab and locate the swim team under clubs and then join it.
Once you have joined it, you will automatically learn the ability to swim, thus will be able to survive dangerous encounters while at sea. Along with that, learning to swim will also help you should you crash a plane while flying it.
Surviving a dangerous encounter is also a requirement for this week's challenge and can be helpful for other challenges in the future.
For more on Bitlife, check out How to get a Boating License in Bitlife and How to become a Pilot in BitLife – Pilot License Test Answers!