Is this a science experiment? I hope you are documenting this so you don't have to do it often. Plastic islands on fresh water will contaminate that body of water with BPA's.
I would get plastic pots with drain holes, use sterile potting soil. That will give your plants at least a chance. I would use Golden Hops. This vine is awesome fast growing...mind boggling fast. It would definitely hold those plastic bottles together well. Within months! Pallets would rot eventually (5 years+) and you should make sure that wood is not pressure treated...not usually but pallets are made of scrap wood that most certainly could be pressure treated with chemicals not good for a pond. Mesh, 1/4 mesh would hold it all together but you have to make it so that it doesn't disintegrate, pieces could get baby ducks and bass and other wildlife tangled. Not cool.
Plant hops starts from 1 gallon (usually what is found to purchase hops is 1 gallon) into 3 gallon black pots. Use mesh to support them on your island. Allow the bottom of the pot to be in the water about a half inch. When you up pot your hops from the store into these 3 gallon pots, add Osmocote extended release 14-14-14 to the 3 gallon pot soil per directions (this will last the entire season) (if you have a season), soak the soil and plant before installing in your island. The mesh will help protect the roots growing out of the bottom from fish, ducks and if you have wildlife in your...pond, mesh over the top might be necessary as well.
I am making an awful lot of assumptions here; this plant, hops, you can actually watch grow. Mass of leaves and stems, vigorous doesn't do it justice. I am worried that you are expecting some ecosystem to develop within a few years, within your lifetime. Not happening.
As a short term 'science project' this could be fun to do. Don't know where it is you live, hops, is a perennial that dies all the way to the ground for winter. The gardener is able to clean up and remove all dead vines and leaves easily, the next spring that hops is back with a vengeance! I have witnessed this plant covering an arbor 100 sq. ft. at least 1 foot deep by May. To continue going up and over trees over fences 50 feet away, covering another 100 sq. ft. getting 2 feet deep by June.
Your project would not require you to 'clean up' the vines/leaves, they would be caught up in the mesh. If they were allowed to decompose at the bottom of your pond...this plant would be considered an exotic invader. I'd need to research the chemistry of hops to make sure it wasn't THAT exotic.
I'd like you to begin to see that whatever we humans try to control will always be artificial. All of our gardens are artificial. Ecosystems take thousands of years to become established. I am serious. There is no way we humans will ever be able to recreate an ecosystem, we just live too short of a time.
I hope this provides some good thoughts that will help you. It would be nice to get more information such as your climate, zone, soil types, fish/wildlife involved, average pH of your soils, the pH of your pond (?), is this a natural pond fed by subterranean water or is this fed by tap water?
You have heard about an island in the Pacific, formed in an eddy by natural currents of the ocean where our oh so lovely trash that floats has accumulated and dunno if this is true, but I heard people are able to walk on this floating island! If I find a link I'll send it to you...this is nature actually doing what you are wanting to do. Century after century, millenia after millenia, plants will find this 'niche' die, decompose, make sort of a soil, particulates from the atmosphere will be added...the plastic will be invisible, held to the center layer of this island, while plants will have taken over the job of buoyancy, roots with air pockets? We are talking a million years...no big deal in nature, big deal for humans with a science project to recreate! Grins! How big is your project? Is this a manmade lake with an unestablished ecosystem? Do you have a winter? What are your visions? Camping out on your island? What prompted you to envision and prepare to make this island? What is your back ground in the sciences? If we got caught doing this in the States we'd be fined. Big time. Depending on the hydrology. Ponds are part of a system. They are not separated from the underground waters, rivers, creeks or our drinking wells. If there were salmon spawning streams nearby, there are huge 'buffer' zones that define what can and can't be done within those zones. Gets very convoluted and expensive.
I guess the plastic comes from India and China primarily. Again, dunno how they know that fact? Sounds, fishy, to me. We will someday be known as Homo plasticus...