Hey there newbie, Vexilar and DeeperSmart both have pretty good products that run on iOS. The Vexilar has an actual mountable transducer and the DS has a castable ball that you can attach to a rod or a stringer.
That being said...I shopped around a bunch two years ago and ended up buying a Lowrance Elite 4 HDI. I put a ram ball between my feet and mounted the head unit on that and used their scupper hole mounting bracket for the 'ducer on my kayak. If you're willing to get creative, you can put any fish finder on your kayak...doesn't specifically have to be hand held or portable. Places like REI and AustinKayak have all kinds of solutions for running fish finders on a kayak that don't have to be permanently installed. If you're willing to meet up somewhere, I have a Humminbird 570P with a suction cup mount that you can have while you make up your mind on what you want to get.
Just another 3 cents worth to make it a full nickel for ya...my next upgrade will be to a side imaging unit...fishing shallow water as much as I do in my kayak, it's definitely more bang for the buck on underwater coverage area.