IPA: /ˈboʊt.fəl/
KK: /ˈboʊtˌfʊl/
A quantity of something that is enough to fill a boat, or a boat that is filled with that thing.
We caught a boatful of fish during our trip.
The word 'boatful' is formed from 'boat' (from Old English 'bāt', meaning a small vessel for traveling on water) and '-ful' (meaning full of). Therefore, 'boatful' means full of boats or a quantity that fills a boat.
Think of a 'boat' that is 'full' — that's what 'boatful' means, a quantity that fills a boat.