I am trying to understand your post. You have a 3 foot diameter plastic tube that is 10 feet long. Would you cut this tube into a shorter length? 2 to 3 feet? Most counties have a depth limit for water features. If it is not sealed it will not hold water and any life forms that are in the water would die. You would be constantly refilling with water which would be impractical. You would need to have a liner or something to seal the tube and contain the water.

Depending on your weather and the thickness of the plastic it might not be able to withstand winter conditions.

A small water feature, 3 foot by 2-3 foot depth, can support several surface/underwater plants and water creatures--few gold fish, tadpoles, water striders, frogs etc. without difficulty.

Yes it is a 3x10 tube, think of the playground tubes or a waterpark tube. I'm not concerned with any county laws.

Is there anything besides a liner that I can use to keep the water from draining out of the bottom ? A foot of sand and gravel maybe ?

I didn't think of the water freezing yet, I will look into that some more.

A foot of sand and gravel is not going to hold in water.

I'm having a hard time visualizing a 3 x 10 tube being made into a water feature. sorry.

The cubic feet of a 3ft dia and 2.5ft lenght is 17 which is 127 gallons and a 5ft lenght is 35 cubic feet which is 261 gallons. This seems like plenty of water, I have seen half and full size whiskey barrel ponds that did not use a filter.

If I do use a liner, can anyone answer some of the other questions in my first post ?

I have multiple small ponds including a half whiskey barrel liner sunk in the ground and two whiskey barrels that never go stagnant. No filters or pumps in any of them. Some in full sun and some in partial.

Most bog or marginal plants are fine in shallow water. I have miniature waterlilies (Helvola, Joann Pring) in about a foot of water.

I would not put a pond under or near pine trees unless you are planning on lots of work to keep the needles out. They will stain the water brown.

It would need some sun depending on the plants you like. I have a 3 foot by 2 1/2 ft black plastic? water trough pond with yellow snow flake, anacharis, water hyacinth, dwarf papyrus, bacopa, 2 gold fish, volunteer toad poles and a frog now and then. It is on the northwest side of the house partially under an oak tree canopy on the left side of the front door in the foundation bed and gets sun from noon until dusk. I move some of all plants and the fish into the sunroom for the winter and don't have to buy new every year. We have cold periods and can have 3 inches of ice on top of the pond in winter. I have left the fish out and used a bubbler some winters but since there are just 2 fish they are happy to come inside. I think you will enjoy a little pond. I have used a small fountain, a bubbler and nothing in the pond. It is very relaxing to sit on the porch and enjoy the fish, flowers, dragonflies and fountain. I enjoy the mating toads to some degree but they get very loud sometimes.