Nouns ending in -eau are usually masculine.
Is marteau masculine or feminine?
lemarteau
The rule behind it
93.1% of -eau nouns are masculine
In context
Tom, passe-moi un marteau, s'il te plait.
Tom, please hand me the hammer.
Je veux qu'on me rende mon marteau.
I want my hammer back.
Je veux que tu me rendes mon marteau.
I want my hammer back.