wanting to get into kayaking and need some advice

Background - I went to the beach (St. Pete’s Beach Florida) for the first time in 30 years this summer and had a great time. Prices to rent kayaks by the half hour are ridiculous so we are wanting to buy our own to take for the coming year. The closest thing to kayaking I have ever done was canoeing and it was awesome.
What I am wanting - a Sit on top tandem kayak that can carry heavy weights(my wife and I make around 500lbs alone) and performs well on the ocean(place we were at the waves were almost non existent. My wife has fibromyalgia and will probably have to be able to take breaks from paddling so I need something that once moving I can maintain by myself. When in the boat with my 7 year old I will outweigh her by about 170lbs so it will have to be able to stay steady even with weight differences. We want to be able to carry snorkel gear comfortably for a few miles while paddling as well to jump to the shell island that is within site.

Needed advice - of the sit on top kayaks I have looked at the ones that keep standing out are the BIC Kalao(because it can hold up to 680lbs) or the Ocean Zest 2 Exp. Can anyone offer any advice on these two kayaks or have other suggestions I am still just researching currently so the more information the better. I would like it to have the ability to add comfortable seats, and if at all possible a rudder and fishing pole holders in case we expand our usage.
Also I could have sworn that once I saw a kayak seat that was also a beach chair but I have been unable to find it again, has anyone seen this before or can offer advice on the seats in general. I am by no means rich or well off but I want to buy quality stuff so I won’t have to replace it in just a couple of years.

All advice welcome I am a newby in the most extreme since of the term.

sorry just reread this by my wife and I are 500lbs alone I mean our combined weight is 500lbs without gear

Have you considered a proper rowing skiff?

Cobra also makes some high capacity kayaks. The following should carry both of you, plus some gear.

http://www.cobrakayaks.com/product/tandem-longreach-kayak-value-pack/

The question is, really, whether it paddles well, and I can’t answer that one. Just giving you another data point.

Rick