Denise
If your extension is just for your boat, (ie not a community or marina project, there is an even quicker fix. I'm not sure the size of your tongue, but for the sake of an example, let us say that it is 3" box steel. Let us also assume that from the actual hitch back to any other obstruction under the tongue, is say 10 feet. That is how much room you have to play with and the size of the extension, so go down to your local steel supplier and buy an 8-10 foot piece of box steel, same size, and from your local hardware store, two U bolts that will hold your extension under the tongue. Also buy a ball to fit the hitch of your trailer and a hitch to fit the ball of your vehicle. About 3 feet from the 'back end' of your extender, mount your ball, and the hitch of course at the front end of your extender. Strap the whole thing under your current hitch using the U bolts, and by giving it a quarter rotation so the ball is facing to the side. This is how you store it. Now in the spring when you need to launch, undo the U bolts, insert the rear ball on the extender onto the hitch of the trailer, and reattach one of the U bolts near the back end of the extender securing it to the tongue. Your entender hitch of course is now 5-8 feet forward of its original position. Hook it to your vehicle and launch your boat. This is a lot cheaper than getting wheels, hubs, tires, and welding it all together, (unless of course this is a marina project) As well the contraption is easily carried under the tongue of your trailer. I have also seen tongue extenders that are strapped to the top of the trailer bed, and significantly longer than the original tongue, same principle though, people clip them on with a couple of U bolts or a single U bolt and hitch ball, whatever works easier. There was a website that had a fairly sophisticated extender which slid out http://www.xtend-a-hitchnorthwest.com/ and another that folded out http://hcgiles.home.mindspring.com/trailermods.htm By the 'cart to haul boat trailer around with' I suspect you are referring to a hand dolly, (although I did see one at Princess Auto, that was electric. That will still not get you the extension that you need to keep your wheels dry, but with an extension mounted to it should work. Remember that the tongue weight on most trailers is no more that 10% and most vehicle hitches max out at 500 pounds tongue weight. Most small boats launched by a dolly would not be much more than 500 pounds, so I would suspect the dolly could hold the weight. Hope these give you some ideas.
If your extension is just for your boat, (ie not a community or marina project, there is an even quicker fix. I'm not sure the size of your tongue, but for the sake of an example, let us say that it is 3" box steel. Let us also assume that from the actual hitch back to any other obstruction under the tongue, is say 10 feet. That is how much room you have to play with and the size of the extension, so go down to your local steel supplier and buy an 8-10 foot piece of box steel, same size, and from your local hardware store, two U bolts that will hold your extension under the tongue. Also buy a ball to fit the hitch of your trailer and a hitch to fit the ball of your vehicle. About 3 feet from the 'back end' of your extender, mount your ball, and the hitch of course at the front end of your extender. Strap the whole thing under your current hitch using the U bolts, and by giving it a quarter rotation so the ball is facing to the side. This is how you store it. Now in the spring when you need to launch, undo the U bolts, insert the rear ball on the extender onto the hitch of the trailer, and reattach one of the U bolts near the back end of the extender securing it to the tongue. Your entender hitch of course is now 5-8 feet forward of its original position. Hook it to your vehicle and launch your boat. This is a lot cheaper than getting wheels, hubs, tires, and welding it all together, (unless of course this is a marina project) As well the contraption is easily carried under the tongue of your trailer. I have also seen tongue extenders that are strapped to the top of the trailer bed, and significantly longer than the original tongue, same principle though, people clip them on with a couple of U bolts or a single U bolt and hitch ball, whatever works easier. There was a website that had a fairly sophisticated extender which slid out http://www.xtend-a-hitchnorthwest.com/ and another that folded out http://hcgiles.home.mindspring.com/trailermods.htm By the 'cart to haul boat trailer around with' I suspect you are referring to a hand dolly, (although I did see one at Princess Auto, that was electric. That will still not get you the extension that you need to keep your wheels dry, but with an extension mounted to it should work. Remember that the tongue weight on most trailers is no more that 10% and most vehicle hitches max out at 500 pounds tongue weight. Most small boats launched by a dolly would not be much more than 500 pounds, so I would suspect the dolly could hold the weight. Hope these give you some ideas.