My boyfriend recently got me a new life jacket for kayaking. It's very comfortable minus the underneath strap that is supposed to go between your legs is always in the way. I don't wear it. I typically either don't buckle it or just leave it hanging (that didn't work, it tripped me.) I'm can't really swim, but I don't think I need the underneath strap. It's not a great fitting jacket but I don't think it will come off. Can I just cut it off? Or just not wear it?

You need it to keep the jacket from being able to come off

The jacket is designed to keep a person properly afloat properly by using the jacket as recommended, that means using all of the straps. By not using it, you put yourself in danger, especially with the fact that you said you cannot swim. The life jacket is designed that way for a reason, just like seat belts in cars. They are necessary for your safety.