I dive the most in the Netherlands.
There are 2 main waters to go to: The Grevelingen (no currents) and the Oosterschelde (tides, so check the schedules)
Watertemperature (now it is around 20°C in undeep water). A 7 mm is good, but with 5mm it would also be doable
What can you see over there: millions of oysters, many crabs and lobsters, small fish, if you are lucky some sepias, snail, seastars...
It ain't like a tropical reef, but it is rather good for the North Sea
The visibility: 12 feet is good. When you go 60feet deep, you can expect some zones with a visiblity of 3 feet, so carry a light with you. Best thing is to go with a buddy and a buddy-line if you are not used to dive in such conditions.
There are plenty of places to dive in both waters (I guess around 150 in both). So there are many dive-shops in the region. In all those stores you can fill your bottles, and in the bigger stores, you can even rent equipement. (suits, regulators, bottles, jackets, (even dry suits if you woul like to)). I'm not sure, but I guess you will pay around €50 per day for a full equipment. Not cheap.
On the most popular diving sites, you will also find containers with a compressor included. Inserting some coins, and you can fill your tank (most of the time up to 200 bar)
In the shops itself, you will be able to fill 300 bar tanks. Both Yoke and DIN are much used in the Netherlands
If you have any other questions, just ask.