There are other coastal towns in Japan called "Mihama" but most of them use 「美」 (beauty) for the first kanji instead, a character that is also read "''mi''" in names.  It's not unreasonable to then conjecture that "Mihama" means "beautiful beach", and that the Mihama in Mie for some reason chose an alternate character when it came time to assign kanji to the name.

There are other coastal towns in Japan called "Mihama" but most of them use 「美」 (beauty) for the first kanji instead, a character that is also read "''mi''" in names.  It's not unreasonable to then conjecture that "Mihama" means "beautiful beach", and that the Mihama in Mie for some reason chose an alternate character when it came time to assign kanji to the name.