The programme, in which six celebs sail around the south coast of Britain, appealed to just 2.81m (12.8%) for ITV1 from 9pm, down 450k week-on-week.
All At Sea was again soundly beaten by BBC One in the 9pm hour, with Have I Got News For You taking 5.24m (23.4%) and comedy panel show Would I Lie To You? appealing to 3.58m (16.4%).
Earlier on BBC One, Watchdog averaged 4.09m (18.5%) from 8pm, up 410k week-on-week.
On Channel 4, Location, Location, Location pulled in 1.92m (8.7%) in the 8pm hour and 298k (1.4%) on timeshift. A new episode of How The Other Half Live delivered 1.71m (7.8%) for the channel from 9pm and 141k (1.2%) on timeshift.
The Million Pound Drop Live appealed to 2.13m (11.8%) for Channel 4 from 10pm and 190k (2.0%) on timeshift. It was followed by Charlie Brooker's You Have Been Watching, which attracted 740k (4.1%) and 89k (0.1%) on timeshift.
The Culture Show brought in 690k (3.6%) for BBC Two from 7pm, followed by the RHS Flower Show with 1.94m (8.8%).
Documentary series History Cold Case attracted 1.64m (7.4%) to BBC Two in the 9pm hour, up 110k week-on-week.
Britain's Greatest Machines With Chris Barrie had 612k (2.8%) for Five in the 8pm hour, then a screening of High Plains Drifter appealed to 1.67m (8.3%) from 9pm.
Overall, BBC One secured victory in primetime with 22.5% over ITV1's 19.6%. Channel 4 came in third with 8% (+1: 0.7%), followed by BBC Two with 6.4% and Five with 5.5%.
Elsewhere, a new episode of The Simpsons pulled in an impressive 622k (3.1%) for Sky1 from 7.30pm.
An extended edition of American Idol, which saw the series winner announced, appealed to 534k (2.7%) for ITV2 from 9pm and 33k (0.2%) on timeshift.
Repeats of EastEnders and Junior Apprentice secured respective audiences of 992k (4.9%) and 406k (2.8%) for BBC Three from 10pm.
BARB ratings data supplied by Attentional