Tap into an Electrical Wire Under a Raised Cottage
Several issues at play here:


First off, everything will depend on the type of wire you are talking
about. If it is UF (Underground Feeder) cable, it might be perfectly
legal. If it is indoor rated NM (Non-Metallic) cable, a.k.a. "Romex",
it is already not to code, so any connection you decide on will not
bring it up to code anyway. To be "proper" in that case means redoing
the entire run.
"Splice" by the way implies it has been cut in two and you want to
re-join the cut ends. Did you mean you want to TAP into it and have 3
wires now?
Assuming it is stapled to the joist the chances that you have enough
slack to be able to cut into it and add a tap line are slim, because
you need extra wire to make the connections with anything like "wire
nuts". What you could do (if you have the right tools) is to put
"ring-tongue crimp lugs" on the ends of each wire then use screws,
nuts and lock washers to attach the rings together, then tape over
them with a layer of varnished cambric tape, then electrical tape
over that.
All of this should be done in a weather tight electrical box with a
gasketed cover. The thing is, the threaded holes into and out of that
box where the cables need to pass through would also need to be
sealed. Here's where the type of wire makes a difference. If it is UF
cable, they make "UF Cable Seals" (search that term) that you can buy at supply houses.
They are a fitting that will thread into the box openings, then the
cable is run through a rubber grommet that is then squeezed down to
seal around the cable to make a watertight seal. But nobody makes a
seal fitting to fit around NM, because, again, this is not something
that would be done legally, so they don't make parts to do it. If you
want to rig up something non-code compliant anyway, you could try using the UF cable seals anyway, then to make sure, seal around the
wires with silicone sealer.