Lionel Messi's having a great week -- like super duper great -- and the celebration's continuing for him on his honeymoon in the Caribbean.

Messi and his new wife, Antonella Roccuzzo, hit up the beach with their 2 sons for some fun in the sun ... and an epic game of Smashball!!!

If it looks like the soccer star doesn't have a care in the world, it's probably because he signed a massive contract with FC Barcelona -- $33.6 million a year through 2021 -- just days after tying the knot with his childhood sweetheart.

Oh and by the way ... she looks smoking hot.

Life is good for Lionel.