I have no idea about the cost, sorry. Others might have recent experience.
My comment is advice to do as much of your own research as you can. Make it really easy for the EC to see this is not just a good idea to get behind but a clearly thought out proposal with everything considered such as on-going maintenance costs (or savings on current heating costs), a realistic assessment of how long the swimming season would be extended and so on. Don’t assume others will see it as obviously a good idea as it might seem to you. Find a few allies! Point out the increase in property values for those who couldn’t care less about having better shared, community facilities.