Hundred

Hundred is a light novel series that received an anime adaptation in 2016.

Set in a world where an extraterrestrial organism known as "Savage" attacks mankind, the only technology capable of combating it is a weapon known as “Hundred", getting it's name from it's ability to transform into many different forms. A survivor of a Savage attack, Hayato Kisaragi is a teenager who is invited to master his skills at Little Garden, a prestigious military academy.

Episode 4
Hayato takes a bath.

Episode 7
Hayato and his friends are at the beach.