Olympic swimmer Andrew Bree failed to qualify from his 100m breaststroke heat despite a personal best time of 1:01.76.
Meanwhile Ireland’s men’s four of Cormac Folan, Sean Casey, Jonno Devlin and Sean O’Neill join Britain in the semi-finals after finishing third in their heat.
Britain ended day one without a medal after their best hope, Craig Fallon, failed to progress in the judo.
Fallon beat Yann Siccardi but his gold medal hopes ended when he lost to Ludwig Paischer in his second contest.
He did win a spot in the repechage where he then beat Younes Ahamdi and Jin Kyong Kim, but defeat to Israel’s Gal Yukutiel robbed him of a chance to compete for a bronze medal as he finished in seventh place.
Both of Britain’s headline sailing outfits overcame a shaky start to end the first day in a healthy position.
Ben Ainslie finished a lowly 10th in the first race of the Finn class but fought back to win the later event, taking third place overall.
Yngling trio Sarah Ayton, Sarah Webb and Pippa Wilson were last at one stage in their first race but ended up with second place, and third place in their second race put them in the overall lead.
Ireland’s Tim Goodbody finished 22nd and then 13th in his races alongside Ainslie in the Finn class.
Britain’s first boxing contender James Degale lifted spirits in the camp following Frankie Gavin’s failure to make his fighting weight with a comfortable win over Egypt’s Mohamed Hikal in the middleweight division.
Hopes of success had been hit by a less than favourable draw and Gavin’s withdrawal but Degale, 22, overcame the former world bronze medallist 13-4.
There was also a triumph in the ring for Ireland as Kenny Egan claimed an emphatic 22-2 victory over Julius Jackson of the Virgin Islands in their light-heavyweight bout.
None of Britain’s three entries in the cycling road race finished the course, but Ireland’s Nicholas Roche and Philip Deignan finished 64th and 81st respectively.
Alison Williamson and Naomi Folkard will be hopeful of being in medal contention after impressive showings in the archery individual ranking round.
The pair both scored 651 in the event to finish in seventh and eighth places, securing themselves favourable draws in the opening three rounds of the competition, which stars on Tuesday.
Britain’s third competitor Charlotte Burgess came 40th after a score of 623, meaning she will face a much tougher tie in the first round.
Simon Terry also finished seventh in the men’s event, with Alan Wills 21st and Laurence Godfrey 34th.
Ireland’s Chloe Magee booked her place in the last 32 of the women’s singles badminton after coming back from a game down to beat Estonia’s Kati Tolmoff 2-1.
But compatriot Scott Evans fell at the first hurdle as he lost a thrilling clash to Germany’s Marc Zwiebler by the same score.
Britain’s Andrew Smith did progress though, beating the Czech Republic’s Petr Koukal 2-1 and will face Zwiebler on Monday, while Tracey Hallam eased past Hong Kong’s Yip Pui Yin.
Siobhan Byrne was beaten in the first round of the women’s individual sabre in fencing, while another Irish competitor, shooter Derek Burnett, appears unlikely to reach the final of the Men’s Trap after he ended the first qualification event in 15th place, with just the top six reaching the final.