When you are wearing a turtleneck sweater and you fart, the fart gasses will travel up your pants, up your sweater, and exit through the turtle-neck opening at the top of your shirt. The turtle neck acts similar to a funnel on a steamboat (aka steamship, steamer).
After finishing a two-pound burrito and leaving the restaurant in his turtleneck, Javier was farting so much he resembled a paddle steamboat traveling down the Mississippi.
The act of taking a hit of weed then going under water and blowing out all the smoke. The bubbles will then pop at the surface of the water and smoke will come out resembeling a steamboat.
Randy: Bill you'll never believe what happened last night at Joey's house.
Instead of placing one's face between a female's breasts, an individual instead elects to place his or her face between a woman's ass cheeks and performs the same actions one would do while motorboating.
Chuck: Hey Zach, what's that brown smudge on your nose?
Zach: Haha, I was just steamboating my girlfriend a few minutes ago and I guess I forgot to towel off