Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
The most resent changes to the Sea Scouts advancements has a new elective that is NOT showing in scout book.
In the downloadable book it is under Specialty Proficiency it reads- level 3 Become proficient in boardsailing, surfing, kayaking, or whitewater rafting/canoeing. On mine is looks to be on page 19 of 21.