Concealable 357 for fishing/hiking? Ruger makes the SP101 in 357 with a 3" barrel. If you can find a well made one and put a Hogue grrip on it, that would be my recommendation. The Hogue grip will give you a full hand grip. At 27 oz., the weight will be tolerable. I think if you went with anything larger/longer, you would sacrifice too much concealability and convenience.
I would not worry at all about the velocity difference between a 3" and 4" barrel with any reloads. Regarding bullet weight, my SP101 is well regulated with 158 gr. 357 reloads (16.6 gr. W296), but tends to shoot a little low with lighter bullets (110's).
I can't recall for sure if I have put 180's through mine-I think I put a few Winchester partition gold 180's through at one point just for grins, but can't remeber for sure. I just dropped a 158 in my SP, and it looks like there is a little room left for a longer bullet. I had always thought the extra weight went into the case anyway, not into OAL of the round. Anyone had any trouble shooting 180's out of an SP101?