Bepe, as was said above, you need to practise moving hips, even better, do it with your eyes closed once you have good muscle tone.
Without going into a 2000 word essay on the physiology of movement and balance there are parts of your nervous system that can detect changes in muscle stretch/tension and put together with your inner ear stability process, if you have those senses well developed you dont even need to be looking at those waves (as we do when starting out) for our bodies to instinctively react to the boat moving beneath us.
This is how I know if I have mastered the boat I am paddling, if I am able to look at the horizon instead of what my boat is about to encounter and keep a smooth paddle stroke.
As for boats, we are becoming truly spoil for choice and the 2nd hand market is growing with the sport
Boats thats come to mind that you should look out for are the...
Epic "V10 sport"
Fenn "XT"
Kayak Centre "Dorado"
Think "Evo"
There are a lot of other boats that claim to be beginner/intermediate ski's, I'll let others with knowledge of them add them if they wish.
I have been through 3 boats in the two and a bit years since I've started paddling and I'm shopping for my 4th.
One thing I'll say is that the iconic brands of Epic and Fenn sell really well and 2nd hand they sell very quickly, if one comes up 2nd hand you need to get onto it right away.
I sold my V10 sport within 2 weeks and V10L within a week of advertising and for very close to purchase price, they hold thier value. If you do pay a little more for one of these boats you may well do better on resale when you trade up.
I'm not saying they are any better than the other boats out there but when the flock are paddling them the sheep tend to follow