I would use what roofers call seam tape (I'm pretty sure it's the same thing
you are calling "splicing tape") and then I'd cover the inside of your seam
with an additional layer of what roofers call cover tape (it's only sticky
on one side) for additional protection over the years.

I built our stream system out of "free" roofing rubber scraps and it has
maybe fifty feet of seams in every place imaginable four years ago, and the
main problem (other than #%&*# chipmunks or mice chewing holes in it during
the winter) has been plant roots (especially cattails) managing to find the
tiniest imperfection in the seams and expanding them into leaks after the
roots die back each winter.

Where I later added the cover tape over the seams where we have cattails,
this seems to have stopped that from happening. It doubles the original
price of making the seam (two strips if tape instead of one) but if you're
going to add strong rooting plants to your pond in the future, I think from
experience, it's worth it for the added protection, not to mention having to
pull the liner or drain your pond later to add cover tape or a patch later.

Oh yeah, and whatever it costs, buy a can of the recommended primer (it's
the same for both types of tape) and use it as directed. That's another
mistake I can warn you of from experience -- gas is a cheapskates poor
substitute that originally cleans the rubber but doesn't help the tapes form
a good molecular bond over time.

Good Luck

RGB
Zone 5 OH