Open water swimmer Moesha Johnson has taken a bashing as she just missed out on a medal for Australia in the opening 10km event at the world aquatics championship in Qatar.

But while it was frustrating to be pipped for the bronze in a close finish around the old Doha port as she had to settle for fourth place on Saturday, the 26-year-old Dolphin at least had the substantial consolation of effectively booking her place at the Paris Olympics.

Johnson, a relative newcomer to the rough-and-tumble world of open water racing, was left smiling: "It just hasn't sunk in ... that was such a hectic race, I was getting bashed out there.

"People on top on top of me and people under me, I just had to hold on towards the end."

After being the race's early pacesetter, the Tweed, NSW swimmer was ousted in the finale for the bronze by Portugal's Angelica Andrea, who finished just over two-and-a-half seconds ahead in 1 hr 57min 28.2 sec, with Johnson timed at 1:57:30.8.

Just up ahead, Dutch great Sharon van Rouwendaal, the 2016 Olympic champion and silver medallist from Tokyo, showed her famed finishing kick to pip Spain's Maria de Valdes and take the gold in a dramatic photo finish in 1:57:26.8, a tenth of a second ahead.

Australia's leading open water campaigner Chelsea Gubecka, who has already qualified for Paris after winning silver at last year's World Championships in Fukuoka, watched the race as Johnson and Maddy Gough battled for a top-13 spot that would effectively allow the pair to join her.

Johnson's selection is still to be ratified but she's now all but certain to be awarded the second 10km spot, while 24-year-old Gough misses out despite fighting back from 28th position to finish 18th in 1:57:51.

Team leader and Swimming Australia's High Performance Director Greg Shaw was impressed.

"Someone like Sharon (van Rouwendaal) would have 100 races under her belt," Shaw said.

"Moe and Maddy only have a handful in comparison ... they showed a lot of toughness today and to have three swimmers - when we include Chelsea - in the top twenty in the world just shows how much depth we have."

In the equivalent men's event on Sunday, Nicholas Sloman and Kyle Lee will also try to seal a place at the Games in what is their last selection opportunity.