That really is a great-looking stamp. The seal seems to have been chipped away a little due to fishery use, sea and beach conditions, but not enough for a reduction in rating. The location of the find sounds remote and adds interest as a back story. The undocumented mark is similar to WP #295, 'MU' within a square (seihokei-mu). To me, the mark in the photo of this float looks like the katakana symbol 'MA', so I'll call it 'seihokei-ma'.