The lifestyle expert also shares her secrets to looking this good.
Followers praised Stewart for the sultry photo, with some labeling it a "thirst trap." "I don't even know what that is," Stewart told ET's Nischelle Turner on Monday.
Turner explained the term to Stewart -- who happily agreed with the label. "That's definitely a thirst trap," Stewart said.
The mogul has posted a few glamour shots on her Instagram before. This one came about because she "just thought I looked great coming out of the pool." "My camera came on backwards, you know, selfie mode, and I looked so nice. The sun was on my face. I thought, 'Oh, that looks pretty,' so I took the picture. It looked good."
"Dear Chelsea, I’m so happy that you like my post well enough to emulate it,” Stewart commented on the pic. "I do think my pool is a little bit prettier than yours and that my facial expression is a little bit more relaxed. Nevertheless you are a beautiful girl and F**KING too young to be emulating me. Continue with your comedy, your humor, and your amazing ability to gather around you political liberals. We need it! Love Martha."
Stewart and Handler have since made plans to get together for an activity Handler suggested.
"I just took up Chelsea Handler's challenge to smoke a joint with her. I told her I haven't smoked a joint since the 1960s. So for me to smoke a joint is a really big deal," Stewart told ET. "We haven't done it yet ... it has to be together, some time in the future."
Before she built her business, Stewart earned money as a model -- but she's quick to say she's still in her "heyday."
"Just eat well, exercise well, garden, climb mountains," she said of her secret to looking this good. "I climbed Kilimanjaro. For goodness sake, I went up into the Himalayas. You have to do that kind of stuff. That's the kind of stuff that keeps one vibrant and young and it's important."
CBD skincare also helps. Stewart, who turns 79 on Aug. 3 -- "I'm finished with birthdays, darling. We're going backwards now," she declared -- has been working for the last six months to develop a line of CBD skincare, which will be out later this year.
"It's called 86 Elm, and this is exclusive to you. I haven't told anybody yet," she shared. "I've been using that for the last six months, and I really think it has helped make my skin even more lustrous and good."
Stewart has been keeping busy amid quarantine. Her new HGTV series, Martha Knows Best, debuts Friday, showcasing the businesswoman sharing home improvements she's done on her farm in New York and giving advice (virtually) to celeb friends like Lupita Nyong'o, Richard Gere and Hailey Bieber.
"I remember when Ellen [DeGeneres] called and she wanted to see the farm," Stewart said, sharing that giving home improvement advice to celebrities isn't out of the ordinary for her. "There's a lot of celebrities living in this neighborhood. Blake [Lively] and Ryan [Reynolds] live down the road, and they've been over. Bruce Willis just moved from this area, but he was here for a long time. I like him so much. He's a really great guy. And Michael Douglas and his wife [Catherine Zeta-Jones], they live nearby. ... Oh the best, Emily Blunt and her husband John [Krasinski], they've been over."
So, on Martha Knows Best, fans can expect some honest advice from Stewart, who knows that sometimes the best way to get through a hard day of work is with a few curse words.
"After 118 dinners and five months of COVID quarantine, you can say anything you damn well please," Stewart declared.
Martha Knows Best premieres Friday at 10 p.m. ET/PT on HGTV.