What is the purpose of putting plants in pots in the water? Some plants need soil to grow, you must use a clay base or clay soil and soak it good before you put it in and also in a pot with no holes if it's going under water, bog plants need holes but cover the holes with cloth or filter medium is what use.
You do not want to use soil in the bottom of the pond or rocks or gravel, it makes it to hard to clean and the water never will get clear, it will stay muddy. a pond will need to be cleaned out once a year.
most people build them out of cinder blocks then put a liner in, then cover them with steeping stones, a boat should work good, I was thinking a aluminum flat bottom
A good liner are tough, my son's dog has been in mine many, many times.
A good filtering system is a must, my is home made and keeps the water crystal-clear.