The maritime world is filled with specialist vocabulary the conventional spelling of which is not always obvious. Boatswain/bosun and forecastle/focsle are among the best-known examples of words that have two accepted spellings, one etymologically correct, the other based on pronunciation. Another source of uncertainty is whether a term is one word or two, or hyphenated, as in sea anchor, seabag, sea bass, seabed, sea-lane.
This list is my attempt to impose a bit of order on the subject. It is by no means immutable, and I welcome suggestions, corrections, and discussion about the choices reflected here.
admiral (rear a.; vice a., etc.)
admiral of the fleet