This thread assumes the premise - validated with discoveries of bacteria, water, or prehistoric evidence of the presence of either - that life does indeed exist beyond our own planet. What your suggesting is the debate about sources of INTELLIGENT life, advanced in technology.
Also, anyone reading this thread thus far......if you haven't watched the recent movie "Life", you need to do it.
One of the theories I like the most is alien life existing on Earth...but underwater: if an advanced life form survived light years of travel to make it to Earth - think of the physics around that - then why wouldn't you just park your ship in about 2,000ft of ocean depth? Surely if it survived amazing distances/speeds in space it should have the structural integrity to survive a little sea water. Pop out at night, fly around, grab a redneck or two for some probing, drop back underwater during the day.
I love space, and our scientific abilities made greater through exploring the cosmos. But the fact that 70% of our planet's surface is covered by water, with depths we cannot fathom (pun intended) and have yet to discover...seems like a fine hiding place. Plus, a great source of oxygen and hydrogen if your life-support or technology requires it.