Hey! Tex here, and as Rodney said I spent the first 15 years of my fishing life in a float tube exclusively.

Your waders are a touch thinner than mine, but the key is what you wear under them. Be sure to layer properly... by that I mean don't just wear cotton socks and a pair of sweat pants - you'll freeze if you do. lol
Here's what I wear, and I fish out of big lakes for hours doing so:
Feet - I wear two pairs of socks... one thinner wool pair, then a bigger pair of synthetic wool-type of socks over top. Don't wear cotton... if your feet get wet at all (sweat, condensation, etc), cotton will make your feet feel very cold very quick. Also be sure not to have anything over your feet too tight (socks, flippers, etc). This will cut off circulation and again - cold feet!
Legs - a base layer of synthetic long underwear, and a pair of fleece pants over top. A tip is to put on the first pair of socks, then the long underwear, then the fleece pants, and finally the 2nd pair of looser fitting socks... tuck your fleece pants carefully into the tops of the 2nd pair of socks (ie. don't bunch them - bunching leads to poor circulation!). This will ensure that when you pull on your waders, your pants don't ride up your legs!

Body - base layer long underwear undershirt. Then I usually wear an athletic (non-cotton) tshirt over top. Finally I'll wear a hoodie and fleece over top, or just the fleece if it's not. Don't forget to bring a rain jacket - forgetting to do so will ensure it rains on you! lol
Fishing can be good on Alta right off the launch area. When you get to the water (I'm assuming there is only one launch that we're talking about), there are a bunch of docks to your right hand side along the lakeshore. I trolled a #12 olive micro-leech with a gold bead head along the shoreline, 10-40 feet off the docks, very slowly. I caught a few fish, one of which was a cutthroat that pushed 20 inches! A floating line can work too, but my favourite trolling line is a clear intermediate sinking line. My preference for this type of line is Cortland's Clear Camo 444, but everyone is different and other lines are good too (Rio Aqualux, etc).
Good luck!

Tex