The Big Freeze, a rebroadcast of an old film on the winter of 1963, pulled in 3m (13.9%) at 5.30pm.
Richard Hammond's hidden camera show Secret Service continued to struggle with just 2.14m (9.5%) for BBC One at the same time.
With Dad's Army following with 2.61m (11.3%), it is a rare feat for a BBC Two show to beat a BBC One programme on a Saturday evening.
You've Been Framed won the early evening slot with 4.28m (19%) for ITV and 196,000 on +1. The last heat of Splash! then attracted 5.24m (21.5%) to ITV from 7pm as well as 185k on +1.
The show's star Tom Daley recognised the strong ratings, earlier tweeting: "Great ratings for #Splash again this week! The semi-final is gonna be fuuuuuuun!!!"
BBC One rival Britain's Brightest, hosted by Claire Balding, took 4.59m (19.3%), despite having a 15-minute headstart on the diving reality show.
However, Dale Winton's 11-year-old Lottery gameshow In It To Win It, airing from 8pm, topped the night with a slightly inferior 5.43m (22.3%) and beat Splash! when the pair clashed for half an hour.
Meanwhile, Casualty matched Splash!'s audience and share in its 8.50pm-9.40pm slot, after which 4.18m (18.3%) caught a Mrs Brown's Boys repeat.
Around the same time, Take Me Out continued to grow, setting a new high of 4.44m (18.2%) for ITV (+1: 194k). Jonathan Ross drew the night to a close with 2.82m (12.9%) and 221k on timeshift.
Elsewhere, The Simpsons Movie was Channel 4's best-rated primetime broadcast with 1.25m (5.2%) and 284k on +1, while Celebrity Big Brother topped 5's night with 1.57m (7.2%).
Overall, BBC One maintained pole position with 21%, ahead of ITV's 17.8% (+1: 0.8%). BBC Two earned third place with 6%, beating Channel 5's 5.2% (+1: 0.3%), and Channel 4's 4.6% (+1: 0.9%).
BARB ratings data supplied by Attentional