I use my PoncVac 3 all the time in our 3' deep swimming pool/pond. It is fabulous.
You can get a longer discharge hose by going to a vacuum cleaner place... or junk store. Just make sure you know the size of your attachment so it fits the vacuum. I like the crevice nozzle the best as it stops the bigger rocks but has lots of suction. I don't use the gunk filter but unfortunately we have to send our water down the alley because our pond is lower than our yard... just the way the original pool was built as a sunken one before we even bought the house.
I have gone through several filters and now make my own out of some similar foam that I sew on my regular sewing machine. Yes, I get in the pond.
Yes, I am careful to keep the vac sturdy & safe from falling in with me.
Yes I use an extension cord.
I have had my vac since they first came out, some 3 years ago. Once or twice a year I vac the entire bottom of my 14' x 28' pond and do selective vacs in between. BTW, I have switched to a new bacteria that is making a HUGE difference in the amount of mulm in our pond. It is Aqua One All-Season Pond & Aquarium Clarifier.
I started using it last summer and can not believe how much less I am vacuuming now. And no, I do not have any stake in the company or any pond store but will be happy to tell you where I get it if you want. I'm happy to answer any other questions.