Jake Paul's fiancée Jutta Leerdam made a splash in more ways than one at an indoor water park on Friday -- turnin' heads with her surfin' skills ... and teeny bikini!

The couple seemingly took a break from filming their new reality show, "Paul American" (we see no TV cameras in sight) ... and hit up the DreamWorks Water Park in East Rutherford, NJ to catch some waves at Skudin Surf.

Leerdam -- an Olympic speed skater -- went solo, proudly filming his boo as she rode the waves, showcasing her incredible athleticism, and drop-dead gorgeous body!

The couple also rode some water rides together, sneaking in a smooch, as they hung with several friends.

Jake and Jutta have been smiling from ear to ear ever since their engagement ... when 28-year-old Paul dropped to one knee and proposed to 26-year-old Leerdam in Saint Lucia.

The 11-1 boxer put a massive 12-carat diamond rock on Leerdam's finger ... as the silver medalist literally jumped for joy when she saw the ring.

The ring also features Jutta's initials and birthstone ... as well as an oval-cut diamond resembling the shape of an ice rink.

Love looks good on these two!