diver deep sea diver dolphin trainer
Another word for water that starts with the letter 'O' is oxygen dihydrogen.
Undermine?Underoos, underling
Aquatic
Aqua
Thoughtful? The only word that came to my mind.. I hope it works
Underage.
h20
Some compound words starting with under include:underbrushunderclothesundercookundercoverundercutunderdoneunderestimateundergoundergroundundergrowthunderdevelopedunderhandedunderlineunderlyingundermineundernourishedunderpaidunderpantsunderpinningunderplayunderperformedunderratedundershortsundershotundersideunderstandunderstateunderstoodundertakeundertowunderusedunderwear
sources
Water under the rock-a!
subway, substitude, substance!
eau meaning water