This is the Hydroman who was using his powers much more effectively than the usual dolt, due to some brain shenanigans or something way back in Avengers West Coast.
How does he go up against the menace of Classic Adamantium Ultron?
Gender: Male Location: Planning to take over the WORLD!
this is pretty cool, actually. hydroman could avoid his blasts and probably get inside him. from there it's hard to say what could happen. increasing pressure (like he did to take out simon) probably wouldn't do much, but his insides ARE at least somewhat vulnerable. johnny took him out and simon also damaged him, so i guess it might depend on what kind of protection ultron's insides have. afaik it's never really been clarified, it's always been more of a plot device weakness. so could he mess with ultron's insides? yes? maybe? lol i wonder if he could freeze his insides by turning to ice? the extent of his power was never really explored either. depending ion his level of control maybe he could slip through whatever protection ultron has in place?
overall i'd tend toward yes--he could take him out or at least do significant damage to ultron from the inside.
i'm not sure he'd get there though. ultron is capable of aoe style attacks, like a wide ranging concussive blast. hm can still be affected by something like that i think. he didn't seem interested in getting blasted by hawkeye's electro-arrow. the more dangerous thing is the encephalo ray though. if he got in close and they made eye contact, ultron has been shown (at least in some forms, including the older ones) to be able to use some mind control. at the least he could put hm to sleep.
so, i guess that's a long winded way to say i can see hm winning some, but i give the solid majority still to ultron. if we went no-limits with hm's powers i'd say he wins for sure, but we're based on his 'smart' showings. still though, a rare unique thread.
Both Hydroman and Sandman were always written to be Spider-Man villains... not some hardcore elementals who could solo the planet if they had a bad day.
They are powerful and versatile, yes. But also dumb and with certain limitations/weaknesses.