Sunil I have driven many posts into our pond bottoms using a post driver { similar to a fence post driver} and/or a sledgehammer { 10 lbs.}. I sharpen the end first . Our ponds have bottoms of clay, rock and dirt. If one place is to hard just move over a few feet. It can be tricky from a small boat but not that difficult. In shallow water {4-5 feet}you can easily use a 10 foot A-frame ladder to stand on. We use the posts as part of our brush pile/structure program. We drive the posts where wanted then tie x-mas trees and other trees and brush to them using small trot line string. They soon sink and we then cut the small rope and they sink.
After a while you build up a good pile and the post tells you where it is .There are a bunch of ways to place the trees so you get the size , shape and location of pile that you want . You can leave the rope on at a length so that the tree is in the depth zone you want. Example- in 12 feet of water use a 6 foot x-mas tree with a 4 foot rope-this places the tree in the 4 to 10 foot water zone. I think you can see the many ways this plan can be adapted using multiple posts and trees at different depths . The options are limited only by your imagination. I hope this helps . I intended to post this before under the structure heading . Thanks ewest