As others have said, sea travel is much much slower. One way sea travel, or repositioning cruises, can be very inexpensive for what you get. If you can wait for the return repositioning cruise months later then it’s still a great value. Last minute cruises also can be very good prices. Www.vacationstogo.com has tons of cruises to look at. Cunard is probably the most well know to do frequent transatlantic voyages. Some are as short as 4 nights I think, might be 5. There are three day cruises up and down the west coast of America. When I cruised in Alaska a few passengers got on at our second port and finished the cruise with us saying it was cheaper than other modes if travel for them to get to the next port. There are one way cruises through the Panama Canal.
There used to be transport by boat from Miami to Key West, I don’t know if that still runs. There are still cruises, but this was basically a regular boat ride.
Edit: Here is an example if repositioning. The one at the top is America to Spain for as low as $679. Including all the food. The link might not work eventually since the cruises won’t be listed if the date has passed.