Lavamancer was the major factor in the Welder.dec vs. Fish matchup. Having a Lavamancer dealt with any current and future Welders. I doubt playing Old Man for the same reasons that Lavamancer already took care of isn't necessary. Lavamancer, in my experience, was the winning factor more times than I can remember. Having a sure way to deal damage to an opponnet with a Curious critter is some good.
Besides, Old Man doesn't fit the manacurve of fish, and isn't nearly as tempo swinging for you as it is against you.
Also, I don't see how you could ever call Old Man better through your own reasoning....
GRIM LAVAMANCER
Pros:
Lover mana cost
Cons:
requires cards in the graveyard
makes you play 2 colors which makes you more suceptible to wastelands
OLD MAN OF THE SEA
Pros:
you can use your opponents welders against them
Cons:
Cost 3 mana
-The lower the cost the better, as Fish is based on tempo.
-Gay/R is far better than Mono-U Fish is in the metagame right now. That makes the Wasteland reasoning null and void. Not to mention bad players will strip your manlands.
-With all the fetching and stripping you do, and Dazing and whatnot, you'll usually have 2 cards to feed the Lavamancer. It not, it still beats for 1.
-Using their Welder isn't nearly as good as killing it. Also, by taking their Welder, you morph your 2/3 into a 1/1 for 1UU. Not very good tempo.