thank you Im.. my lake never be calm always.. its just opposite to sea.. lots of waves and wind.. but i choose the time of tides.. and to turn a foam board hull in to rock, in need you can mix french chalk powder with acrylic wall emulsion paints, is water based, in a diluted manner with 50 % water.. then pour inside spread all around.. leave to dry.. when dried, it will turns in to a rock like thing.. and here i am not using soft foam, but it is stiff.. but not too much.. but when formed, it becomes a stiffer and stronger structure.. and lasts for a life time.. today, i am able to buy PU paint materials are so great, scratch proof, gives a great rate of finishing.. but all in reasonable cost, as shop owner is happy to give me in loose quantity.. as 50 ml.. 100ml.. so happy..