I took my commercial check ride in a seaplane.
It may not be the most practical rating for those who won't be using it much, but you'll still learn a lot, and it's worth the experience. Personally, I've found that flying seaplanes is a lot of fun.
The concept of the glassy water landing, and landing on water in general, teaches you about landing on land, too; it's about awareness, and adds a dimension to your flying that you'd be without, if you didn't have that training.
Any training you do is a good investment. A lot of pilots lose their drive after their private. Continuing to challenge yourself by seeking ratings and higher certification is one way to keep that drive going, and maintain your interest. It's also a way to help maintain your proficiency, and while expensive, is always a worthwhile investment.