TileIndex tile = whatever tile the ship is;

DiagDirection direction = whatever direction the ship is headed;

 

TrackBits bits = TrackStatusToTrackBits(GetTileTrackStatus(tile, TRANSPORT_WATER, 0)) & DiagdirReachesTracks(direction);

Track track = RemoveFirstTrack(&bits);

TrackdirBits trackdirbits = TrackStatusToTrackdirBits(TrackToTrackBits(track) & DiagdirReachesTracks(direction));

Trackdir trackdir = RemoveFirstTrackdir(&trackdirbits);