The couple, who are rumoured to be preparing to tie the knot, enjoyed their first outing with baby North West in Malibu, reported Us magazine.
Kim, 32, took North, who was born five weeks early on June 15, to her stepfather Bruce Jenner's new beach house, where her entire family were celebrating the occasion.
"Kim has taken North on a few play dates, but she can't do it with lots of people since she's still so small," a source said.
Her younger half-sisters Kendall, 17, and Kylie, 15, were also seen swimming with Kylie's boyfriend Jaden Smith.