I did quite a bit of reading on this in my first pregnancy and came to the conclusion that I would not be going anywhere near water for anything in the world, once labour began and/or my waters broke.
The science does not at all support water birth and it carries direct risks to the bang of drowning, inhaling or swallowing of water and of infection from the faeces contamination in the pool. No mammal that can spend any time on land ever gives birth in water - seals, walruses etc all whelp on land. Only whales and dolphins remain in water, simply because they are never on land. There is no evolutionary or other reason for human babies to have a diving reflex. A pool heated to the temperatures used in these systems is also at a perfect temperature for bacteria to multiply to a pathological level. There are studies showing all water pools in use in labour and delivery are contaminated with fecal matter. At no time in human history was water birth practiced until it was piloted in the Soviet Union for pain relief at a time when the failing regime could not afford to supply medication to labouring women.
At an absolute push, I could see a rationale behind early labour before rupture of membranes in water for pain relief but once membranes are ruptured the risk of infection are far too high for me to tolerate. And never ever for delivery.
If pain relief is the primary concern, I had excellent personal experience with an epidural.
Someone above mentioned a calm baby at the moment of birth as a positive outcome of water birth - for me a calm silent newborn is a huge cause for concern not celebration. A gusty roar helps the lungs full and the blood get oxygenated - anything less than that would have me panicking like crazy.
So, you did ask for cons and opinions, that's my contribution. I know I'll get hounded on here for it.