It depends on your personality I think.
Personally I would book the cruise. You might not even fall pregnant then, and then you would have missed the cruise.
But if you did book it, you would have to accept that you might subject yourself to morning sickness on board a ship, or less control over your daily schedule or diet than you'd like, a slightly higher risk of having to cancel if anything went wrong with your pregnancy, it being more difficult to get appropriate medical attention if you need it etc.
If this is going to freak you out, don't do it! - you will be miserable. If you are the sort of person to take it all in your stride, the chances of it going badly wrong are relatively low.
However, insurance may be a problem because Diabetes is a naughty word, as is Pregnant, and the two of them together...... well.....!
I went on a (flat, track) ski-trip when I was six months pregnant last time round: it was the best thing I could have done. But I wouldn't have got insurance cover even if I'd have looked for it and would have had to accept responsibility for anything that went wrong. If it had and I'd have looked round for the nearest person to sue, that wouldn't have been fair.