You will buy a good middle size electric boat cheaper than you can make one. Here is something typical from googling rc boat: http://www.gadgetgalaxy.com.au/Radio...ol%20Boats.htm
You could also try ebay.
Buy one in your price range. See if it is something you enjoy and then scale up from there.
If you go to Youtube and search for fast rc boats or something similar you will find all manner of craft. Some of them are 6ft long or more.
If you go for fuel powered then the noise can be a problem if your pond is not secluded.
There may be an RC club for planes and boats in your area. These can be a rich source of low cost components as people upgrade their fleet.
Operating them is fun for a while but you will get more satisfaction from building and tuning. However buy something to get a feel first and build up from there. You need to develop a source of components that are priced well and delivered reliably. I buy most stuff like this over the web these days but you need to know what you are looking for first. Building a decent 6ft boat could set you back USD1000. Parts like drive leg, motor, RC units, servos etc are expensive so costs go well beyond the cost of materials to build a hull.