This is how I read this. The well is God - the Fountain of living waters (see Jeremiah 2). The mouth of the well is the mouth of God, and the waters thereof are His commandments and statutes (His ways). They were perceived as being bitter until Christ, the Tree of life, was cast into the well, into the pit, into the belly of the earth, and thereafter the sweetness of God's ways were made known. The twelve wells are the twelve tribes of Israel - in spirit and not according to physical geneology (see Galatians 3) - and the 70 strong trees are the elect in the end times - who are a part of the 7 churches in Revelation 1-3. "I am the Lord that healeth thee"... "By His stripes, we are healed." (Isaiah 53:5)
The other interesting thing about this passage is the time stamp. The Israelites came up from the Red Sea, where only a few people passed through (that being Israel) while many people were flooded out (all of the Egyptians that pursued them). It's very reminiscent of Noah's flood (Genesis 7), where only a few survived the flood. So the Israelites come up from the flood, three days pass, and then they have this sign of the Christ at the well of Marah. Peter says in 2 Peter 3:8, "one day with the Lord is as a thousand years, and a thousand years as one day." The interesting thing here is that Noah's flood occurred about 3,000 years before Christ was crucified on the cross. Three days from the flood at the Red Sea to the well at Marah, three thousand years from Noah's flood to the crucifiction of Christ. I think it's pretty neat that it works out that way. God bless.