An Arizona fisherman enjoying a sunny day on Roosevelt Lake made an unexpected catch last week: a 5-foot-long rattlesnake.

The venomous serpent silently slithered aboard the boat unbeknownst to Mark Dalton. It wasn't until the fisherman got up to change his bait that he spotted the giant creature basking under the sun at the front of the small watercraft.

"[We] went back into a cove in the southern part of the lake," the Tucson native explained to Fox 10, noting that the snake was wet. "[I] was probably there for about half an hour when I went to the front of the boat, then went back to change one of my lures and had a big surprise."

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At the time, Dalton's boat was about 30 feet from shore in waters up to 10 feet deep.

Dalton made a point to remain calm and make slow movements. After snapping a few photos of the creature, Dalton carefully picked up one of his fishing nets to gently guide the snake — identified as a western diamondback rattlesnake — back to the water.

"I just wanted to be able to push him out of the boat and not really hurt him or irritate him too much because he looked like he just came out of the water because it was wet back there and he was sunning himself to get dry," he told the news station.

Mark Dalton says a 5-foot-long rattlesnake slithered aboard his boat earlier this month.

Mark Dalton says a 5-foot-long rattlesnake slithered aboard his boat earlier this month. (Courtesy Arizona Game and Fish Department Mesa Region)

Fortunately, the snake didn't need much convincing nor was it aggressive. Minutes later, it slid back into the lake and out of sight.

After hearing Dalton's story, the Arizona Game and Fish Department Mesa Region shared photos of the huge snake on Facebook to warn those who plan on visiting a local body of water this spring and summer.

"Fishing in Arizona… You never know what you might catch," the agency wrote, prompting a slew of responses.

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"They are pretty good swimmers. Keep calm and get it off the boat. Good job," one Facebook user commented.

"Awesome! Makes for a rad fishing story!" another replied.

"Way to go, Mark! Thank you for safely relocating the snake and not killing it," a third user praised Dalton.

The game and fish department echoed the user's praise, informing those who suggested he should have killed the creature that they were wrong.

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"Boating safety includes not putting yourself at risk – whether that’s risky behavior such as boating under the influence, or killing a rattlesnake rather than safely removing it with a net, as this person did. There was no need to kill the snake in self-defense, as it wasn’t acting aggressively, and everyone lived to tell the tale," the agency added.

This isn't the first time a rattlesnake has shocked boaters with its swimming skills. In July. a timber rattlesnake shocked a family when it slithered toward their boat on Fontana Lake in North Carolina. A 14-year-old girl later shared footage of the scary scene.