If I have multi-engine land, can I learn multi-engine sea directly?
As shown on page 77 of this FAA document, the various tests needed for an AMES (air multi engine sea) rating depend on the ratings you already hold and your area of operation. Depending on your area of operation, holding an AMEL (air multi engine land) rating can allow you to skip most or all of the required tasks for an AMES rating. In many cases, an AMEL rating is actually better for getting an AMES rating than a single engine sea (ASES) rating will be. So in short, you do not need an ASES rating first, but having one can make the process of getting an AMES rating easier in certain areas of operation.