Mare
means sea in Latin.
Galileo (1564--1642),
himself suggested they were bodies of
water though he probably eventually realized they had to be plains
(HI-140).
The names of the maria
(at least the largest ones???)
on the near side of the Moon
were given by
Giovanni Battista Riccioli
(1598--1671)
(see
Wikipedia: Giovanni Battista Riccioli: Work concerning the Moon).
But the name mare is actually good.
The mare are frozen seas
of lava: they
are lava plains and are dominated by
basalt rocks: they are
silicates rich in iron,
manganese,
and titanium
(Se-451).