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CoreOfimBalance(COB)
The cutscene didn't suggest any timelapse between the last scene before the detonation. In fact, it clearly suggests the contrary. So, if he took it from that distance and barely survived it, his durability is at best city level at the end of the first game. If he would have tanked it while remaining completely unscathed, it would be
at least city level. There's a difference. And since he is much more powerful on the second game, i would say that his current level is unclear.
In the fight with James, he was defeated by being consumed, and to do that, James had to slow his Regenerationn rate by decapitating his shapeshifted arms, which made he lose massive amounts of biomass. If he kept stabbing or decapitating him he would just be unable to regenerate temporarily. Alex can't be killed by physical attacks.
About the water, it was an example to explain why Alex can't run out of biomass. He is made of it, running out of biomass is the equivalent of disappearing. Sorry if i couldn't make it coherent enough.
His Regenerationn is cell replication. And indeed, it requires time, but we should keep in mind, that he is a virus genetically enhanced. It reproduces at a faster pace if there is enough biomass. In other words, the more biomass Alex stores in his body, the faster his Regenerationn is. It works the other way around, though. The less he has, slower it is.