What counts as a sea-service day
A day is 4+ hours underway on a vessel. 8+ hours can count as 1 day; you cannot accumulate more than 1 day in any 24-hour period. Time at the dock or at anchor without machinery in operation generally does not count.
Recreational vs commercial
Recreational sea time on your own boat counts — provided it is documented and signed by a witness who can attest to the dates and the operator role. Commercial time on a USCG-documented vessel can be verified by the owner or master.
CG-719S: the small-vessel sea-service form
The form requires vessel name, length, gross tonnage, propulsion, the service period dates, route (inland / near-coastal / ocean), capacity served (master, mate, deckhand), and the signature of the owner/operator certifying the time. The NMC rejects forms that miss any of these fields.
Near-Coastal vs Inland time
Inland time accumulates inside the 33 CFR Part 80 demarcation lines (bays, sounds, Great Lakes, inland rivers). Near-Coastal time is service seaward of those lines, out to 200 nm. Near-Coastal credentials require 90+ of your 360 days on near-coastal waters — Inland-only sea time will not qualify.