Shalom - latest floating pond project: bought a preformed small unit used to connect 2 ponds and cut off the spout end and drilled holes in the body for the water and used great stuff under the edge and on the bottom for floatation. in the holes I drilled on the underside, I put in a couple of wire ties for future use to anchor the island. On the top edge I used great stuff again, partly for floation and also to form island edge by adding a mixture of sand, ome dirt, and pea gravel and pressed that into the great stuff as it begins to expand (wearing latex gloves) and it gives a nice edge - a bit light cream color for awhile, depending on the color of he mix and how much of the mix sticks, but the sunlight will darken it. The first floating island made a coule years ago has darkened well.
I did nto get an evn or balanced coating of great stuff on the bottom so balnce was/is a concern, I ended up peeling off some of it and have used a couple of small rocks for balance - time will tell as the plants grow. Hopefully the couple of wire times hanging loose on the bottom can be used to add floation materal if needed.
The problem that may be forth coming is did I use enough floation for when the plants get big ( I have some iris in the island) and balance - I had to use a couple of small rocks to help balance and they look okay.
in the island, I have, a dusty miller, verigated chameleon, Iris, Pickerl rush, horse tail rush, dragon tail, a ground cover that has very samll round leves, (doing quite well), impatiens, marigolds. New plants added, not sue how they will do are aquatic forget me not, fain Lilly, Water willow, coleus, Lucerne Blue Eyed Grass( doing well so far), Moonstruck mountain lilly, hosta - we'll see how these fair in the hot summer sun. I'm going to get some shrimp grass, con tail, water nympth and maybe 4-leaf clover to added.
Plan is for the island to be all perennial.
when I plant, I shake off the soil, leaving some soil, then wrap the plant in batting with a miracle grow plant spike, remove about a 1/3 or so of foilage and into the batting of the island - this also keeps the weight down requiring less floatation
Tip - be sure to keep the crowns of the plants above the water level, some may require more height than other plants. This is why I conveted my pond's iland to floating wa so that when it rained and the water rose some I won't have to go pump it down to save the flowers and it also allows about another inch of water level, which in a Koi crowded pond should also help as well.
Tired typing, hope this helps - if it can float or be made to float it may be an island in the making.