Ivanka Trump's surfing enough this week to consider going pro ... 'cause she was back out in the ocean off the coast of Costa Rica, sporting another bikini.

The First Daughter hit the beach with a few pals ... walking into the crisp ocean blue with her board under her arm -- ready to carve up a few waves.

IT's wearing a small black two-piece bikini this time around ... a lot like the orange number she wore earlier in the week during her beach afternoon with her model sister-in-law, Karlie Kloss.

Trump showed great form with her paddle out ... making it over the crest of the crashing waves.

As you know ... Ivanka and her husband, Jared Kushner, have been down in Costa Rica for at least a week now -- and, Ivanka's spent pretty much everyday surfing off the coast of the Central American country.

Ivanka showed off her backside in a tight one-piece that was riding up when she went the first time ... drawing as many eyes as her solid form atop the board did.

Like we've noted before ... there doesn't seem to be any Secret Service around Ivanka -- though as President Donald Trump's daughter, she does receive SS protection.

Unclear how much longer Ivanka's going to be on vacation ... but, given how much fun it looks like she's having, she may never want to leave Costa Rica!