Liev Schreiber Diving in with GF Taylor Neisen on Beach Vacation

Liev Schreiber is diving in deep with his new gf -- taking a sporty and sexy vacay down in Costa Rica.

Taylor Neisen, an ex-Miss South Dakota, was rocking a small blue bikini while she and Liev played frisbee and jumped around in the surf. The couple's been seen together since last fall ... when they were spotted out in NYC. Good move heading South for the winter.

The "Ray Donovan" star and Naomi Watts broke up in 2016, but they've successfully continued co-parenting their 2 sons -- who are on the trip with Dad and 26-year-old Taylor.