OK, OK, I'll be the bad guy out. I did this with our old 5er due to our local CG would allow us to "circle the wagons"...Literally! For 7 years, we'd stay from mid June to mid July and I needed the extra length to reach the post. BUT...
It has to be done right!
I've hooked up many, many submersible pumps on oil/gas platforms over the last 30 years and we've always used the following 3M Scotchcast kit for the motor lead splices. When done right, they are completely waterPROOF, low resistance and durable; and only ~1 1/2" OD. IIRC, I only saw a 2 volt drop after splicing the original power cord to a 25' extension cord, but it was more like a 8 volt drop when plugging them together (after some age...which caused me to make the splice). If you decide to do this, please PM me and I'll gladly walk you through it step by step. Naturally, the crimps are very critical, as well as taping the butt connectors and mixing/pouring the epoxy.
http://www.shop3m.com/80610981567.html?WT.mc_ev=clickthrough&WT.mc_i d=shop3m-AtoZ-Scotchcast-In-Line-Splice