They are special special cables. As I recall for low voltage and temp (under 100C) we used cables that were polypropylene primary insulation with wither high density poly or nitrile rubber outer jacket.
What is critical is the long term moisture absorption of the insulation, make sure that they are rated for that service.
Only use high quality submersible power cables, the high quality submersible cables have the individual "cables" bonding into and forming part of the outer protection sheath, not individual insulated cables laid-up and then having an outer sheath placed "loosely" over the individual cables. Fully bonded cables have a distinct advantage that in the event the outer sheath gets damaged water can't wick down along side the un-bonded cable to the pump terminal location resulting in an unhappy or disastrous electrical failure.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
Honestly, "rubber" means little anymore, because actual rubber is quite scarce. I'm not a material/chemistry guy, so can't have an intelligent discussion on the rubber-like compounds used in cable jacketing today however. I definitely would lean toward plastics as opposed to rubbers though. Look at the classifications and ratings of the cable. PE jackets have different characteristics than vinyls do, and are typically lighter. Your cables probably won't experience any hard repetitive flexing, so rubbery jackets weights wouldn't be present, and necessary to accommodate. It might sound crazy, like a line from an old movie, but the advancements in "plastics" (used in cable insulation and jacketing) have been substantial in recent years.
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