You want to have a pond and you decide to have a fountain, UV etc, but you have no sockets nearby, what can you do?
According to the building regulation "Part P" which covers some electrical work, gardens are classified as "special locations" CLICK ME for a pdf version of part P
This means you have to have a part P registered person install an RCD protected socket. (Adjacent your pond in this case) They will do this using either steel wire armoured cable or conduit. This is to prevent you from accidentally damaging the cable, and possibly killing yourself.
Under no circumstances should you "Try something that should work" Because today you know where the unprotected pump cable is, next week when digging the garden by the time you remember, you have just sliced through the cable, potentially electrocuting yourself, or worse still, some one else. That?s why the supply cable must have mechanical protection.
Cable damaged or too short?
If you damage a pump cable you are better off buying a new pump. Pump cables are sealed inside the pump so can not be replaced. A pump may cost "a few pounds", but a life is priceless
You can get underwater use cable connectors but often people use the wrong cable so they leak or connect them wrong, so I would not recommend them. If you are trying to extend a pump cable because it is too short, don?t. Move the supply socket closer to where it is required.
Not Mentioned: But worthy of note:
There are available "external use electrical socket kits" That come pre wired with steel Wire armoured cable Using one of these is said to get round part P
Pumps have 10m long cables, that is because of an EU regulation. Pump cables are made to withstand water, not garden implements. (And rodents)
If you have lights or a pump that has a transformer, if that transformer stays indoors (Not just plugged in, indoors) the cables to your lights or pump will only have 24 volts at most, so will not require steel armoured cable.
What you actually do is your choice.
Remember this:Electricity has no prejudices, it kills anyone.
There is more to a pond, how it works, how to look after it. Please look out for other articles for your guidance