Hi there, most of the US lines relocate some of their ships to Europe during our Summer, so you can get really good deals on these relocation cruises.
You will find Azamara, Celebrity, Crystal, Holland America, NCL, Oceanea, Princess, Regent Seven Seas, RCI, Seabourne, Silver Seas, as well as 'local' lines, Cunard, P&O, Fred.Olsen, MSC, Costa to name but a few.
As for health insurance, I cannot help other than to say that if you have Worldwide Cover with repatriation & cruise cover, then you are already on a good start. Perhaps checking with a local TA, or calling a few insurance agents would be the way to go.
On the whole, over here our travel insurance, even with medical conditions, is quite reasonable, UNLESS we touch any US territories, when it rockets