Hi - That's where I'm from. Don't live there right now. The general suggestions everyone else has offered are mostly good. Here's some Mobile-specific stuff.
First, by this time of year, daytime cruising and "out in the open" searching is pretty much done. Road cruise early a.m., or near and after dusk. During the day, you'll have to look under tin, logs, boards, etc., and still won't do as well as you'd have done 30 days ago.
There are some Wildlife Management Areas, like Boykin, a little upstate. Gulf State Park and the Bon Secour Refuge over in South Baldwin can be good walking areas. Interestingly, in Mobile, if you find a King, it'll be a Speckled, but in South Baldwin it will be an Eastern (chained pattern). The Kings on Dauphin Island are also Easterns. Chickasabogue Park is really close to you if you're in Mobile. There, you have a little bit of several types of habitat - pineywoods, bottomland hardwoods, swamp, oldfield second growth, and an upland stream. You'll probably have to get off of the trails to see anything.
Wherever you go in South Alabama at this time of year, unless it is in the city, if you are outdoors you can count on getting chewed up by yellow flies, deer flies, and horse flies. Early and late in the day you can add skeeters to that lovely mix. And if you're anywhere near a salt marsh, those wonderful blood-sucking midges (called gnats down here, but they're really midges) will join in the fun too. That makes road cruising at night kind of an attractive option for the next few months.
I haven't done much road cruising around Mobile, well, not intentionally anyhow. I guess I would drive out west by Big Creek Lake or up near Citronelle and look for secondary, preferably ditchless, roads.
If you have a boat, you can see a lot of watersnakes up on the Alabama River Cut-off. Much more than in the delta. But watersnake spotting was also better a month or two ago.
Finally, I hate to disagree with other posters, but I'd have to tell you that, in Alabama and this part of the South in general, I'd start with the assumption that unless you know land is public or you have specific permission from the owner to enter, don't go hiking around on that land. I wouldn't rely on the absence of "no trespassing" signs or the absence of a fence as an indication that you are welcome. I'm not giving you a legal opinion here, I'm just giving you some practical info as a person who has lived down here all of my life. No one is going to get too cranked if, while road cruising, you get out and take a picture or look at a snake near the side of the road so long as you don't cross a fence or go past a "no trespassing" sign or stand in someone's front yard. But by the same token, if you go traipsing off into someone's woods or fields, even if there aren't any "posted" or "no trespassing" signs or fences, well, it can get a little uncomfortable dealing with some of the landowners.