I'll chime in on this, as we get this question very often. Or at least we did pre-covid.
Depending on what part of Grand you left from and what part of the sister islands you ended at, the distance can be as much as 117 miles from West Bay to the Brac's east end, and as short as 66 miles from Grand's east end to Little Cayman's west end. Your voyage would probably depart from one of the marinas in North Sound, so let's call it 100 miles from Yacht Club to the Brac's west end. 100 statute miles is 86 nautical miles.
Boats can and do make the crossing routinely. With someone to serve a local contact, you'd like be able to find a fishing boat or some other small vessel to make the trip for you. With a super fast boat and calm seas, it could be done in as little as a 2 hours. But at 43 knots (there is one or 2 boats here that can sustain that speed,) this would use an enormous amount of fuel, something on the order of 300 gallons or even more. With marine fuel around $5 CI a gallon, you can do the math easy enough, and realize it's cheaper to hire the Cayman helicopter to fly you at very low altitude. (They used to offer this as a service.) With a reasonable speed of 20 knots, you're looking at closer to 4.5 hours. Our boats burn around 20 gallons per hour at 20 knots, so 90 gallons of fuel, or about $560 USD in just fuel, without paying for the boat or the Captain's time. If you're paying for a one way-transit, consider you need to pay for that person to transit back to their starting point, so double the fuel costs, and account for this person's time. The cost of the boat and the crew is relative to the size of the vessel of course.
But all of this assumes calm seas. Anyone who makes the transit from one island to another always looks at the weather, which of course is impossible to predict long term. Rough seas will reduce your speed drastically, in addition to just being not pleasant. As I look at the weather today, the seas are around 1 meter, and you could not pay me enough to do this crossing in a small boat.
The short version, is there is a reason the sister islands don't have a ferry service. Take a plane.