Assuming you have a gps unit available... concentrate on finding a fish finder that has adjustable sensitivity levels. This will let you fine-tune your unit to see the bait pods AND fish (knowing which you have under you is helpful).
Knowing the surface temp of the water really isn't important, so don't even make that a purchasing issue. When anyone is looking for a particular temperature zone, that's when the fish are deeper and you're looking for the effects of the thermocline. A surface temp gauge on a fishfinder won't help you with that.
A paddle wheel speed gauge will work accurately when the lake is dead flat, there's no wind pushing the boat and there is no water current. How often does that happen? I have yet to see a day like that on Winni. On a normal day, a paddle wheel is going to vary in speed every time you go up and down a wave, when the boat rocks side to side due to waves, when you're going against or with the wind and/or when there is any water current working with or against the boat/engine.
The cheapest way to get a reasonably accurate speed of movement across the surface is a gps unit. A handheld is the cheapest of those...
The best way to know the speed your lure is moving is a submersible temp/speed probe. You're looking at 400 to 800 bucks for that though.
If I were limited to 300 buckeroos to get going for the opener, and I DIDN'T already have a gps available, here's what I'd do. I'd buy a good handheld gps and a cheap fishfinder. My reasoning? I feel that I rely on my gps more than anything. Speed is crucial AND the gps lets me create routes and mark hotspots of where I've caught fish so I can return. I think being able to return to a hot spot or follow a route is more important than seeing fish on my fish finder. The biggest thing for a starter fish finder is actually being able to get an accurate depth reading. This will allow you to see and learn the bottom contours that will hold bait and therefore trout/salmon. You also need to know how deep you are to manage your line depth. (Nobody likes getting hooked on bottom.)
Honestly, there have been days on my boat and on others I have been on that our fish finders were not working and we fished solely using the info from our gps units. Luckily I have Navionics on mine, so it also shows me depths... but as long as you had a saved route or some waypoints to go by, you could continue to fish without fear of getting too shallow.
Lastly, I use Eagle products and have had great luck with them. Good prices on quality equipment. As a backup I have an eTrex handheld gps unit. (I think it was like $150? but provides all the info for speed, location, routes, waypoints and mostly... how to get home if it turns foggy or something.)
Slipknot is right on.My,up front fish finder is an eagle seacharter 500c.I paid 425.00 for this unit last year.It has G.P.S.It should be cheaper this year. I put the navionics chip in it.I found i liked it so much that i just use that one for way points and underwater topo mapping.I have a lowranceX107 in the back of the boat,the graphics are vastly superier to the eagle,same price range but no g.p.s.I must say again,that navionics upgrade made an unbelievable differance.I am glad i bought one that i could upgrade .Polebreaker
