Not included as part of a rating. In some states, the aircraft has to even carry a marine rego number as well, or at least they used too anyway.
Spend the time and money to do the whole thing fairly well on consecutive days, too. Will help you heaps, especially with things like engine failures in confined areas while trying to assess the wind/ current/ traffic both in the air and on the water/cockpit checks (gear - if fitted/ ie; amphib)/and the favoured glassy water dead engine approach, all rolled into one approach!
Will definetly keep you on your toes.
Once again, relax, learn, and enjoy!
Go for it!