Blue Water Sailing

Bluewater sailing is a type of ocean cruising: it refers to long term open sea cruising, for example passages (ocean crossings). Bluewater sailing implies a lack of support and requires a certain amount of self sufficiency, since you're away from land for long periods of time. An example is crossing the Atlantic.

Bluewater sailing is a type of ocean sailing that requires a lot of extra skills and equipment. Crossing an ocean requires a lot of prepping and planning, and maybe even a different boat
Blue water sailing is one of the most advanced types of sailing.

It's hard simply because you have to be self-reliable. You're out on the open ocean alone, sometimes for two or three weeks. It can get pretty rough. If anything breaks, you need to be able to fix it. If something goes wrong, you need to solve it. The stress can be intense, and plenty of people break down because of it.

It also gets challenging with the size of the boat. The smaller the boat, it becomes more challenging.
I sailed in a Swan class, 37 feet boat of 1970 vintage. It was refurbished and used to go around the earth in 1985-87 by Indian Army team.

After that it was used till 2014 for training and smaller cruises and crossings. I sailed from Kochi on western coast of India to Laccadive Islands in Arabian Sea. It was about two weeks of sailing and was an excellent exposure and learning to enjoy the bounties of sea.