Hi Joe
I am not going to recommend a particular kayak - not sure what sort of experience you have, but just point out two basic choices and what to look for.
There are a few basic articles here
http://www.kayarchy.co.uk/html/01equipment/002seakayaks.htm and
http://www.kayarchy.co.uk/html/01equipment/010otherkayaks.htm and
http://www.kayarchy.co.uk/html/01equipment/011otherkayaks.htm that might help
The boat types most suited to gentle coastal / estuary touring are Sea Kayaks, Crossover/ touring kayaks and Sit-on-tops. Sea kayaks are sleek, fast, built for the job - but to get the most out of them you need to learn how to use them
- and they are not cheap. Crossover / tourers are basically long general purpose kayaks with one or two storage hatches. Not as fast as a sea kayak but usually more stable. Sit on Tops don't have a 'closed deck and cockpit. i.e. you sit on them rather than in them. Easy to just hop on and go. They are usually very stable, very popular and cheap. Purist kayakers tend to 'poo poo' them but they are great for easy day tripping in fine weather and good sea conditions. They also appeal to those that hate the 'trapped' feeling of a closed cockpit kayak.
I will leave you to choose what type of craft might suit you best
To keep in a straight line - that will come with practice, anyway - but generally the longer the boat the faster and straighter it will go. Boats with minimum 'rocker' and long water lines will go fastest / straightest too. The features that you are not looking for i.e. Maximum manoeuvrability (i.e. turns easily) = short with lots of rocker - the ideal whitewater boat.
Sea Kayaks usually come with either a 'skeg' or 'rudder' to help them keep in a straight line when conditions (wind and tide) make that difficult. For example My wife, who doesn't really enjoy the technical bits of kayaking, has a Prijon Seayak - this is very stable for a Sea kayak and has a rudder that allows her to keep it straight or turn it without having to rely on edging her boat on the side and using advanced techniques. Rudders are very popular in Europe but less so amongst British sea kayakers (there is a lot of purist snobbery amongst kayakers in the UK. Shhh don't tell anyone I said that
Just one last thing - before you venture too far away from the shore or a decent landing spot, make sure you get some basic understanding of tides and wind etc. You know - the normal H&S caveat
Hope this helps. Message me if you want to chat more about it
Graham