Nice clear and complete stamp of WP #54 'Ten' 天 which means sky or heaven.  The stamp is large and show all points.  It's one of the roof series of marks.

This float measures 3 1/4 inches in diameter and is completely frosted.

Beachcombed on Kunashir Island, Russia.  Kunashir is one of the four major islands in the Kurile Islands just north of Japan.