The weather in the Atlantic can be pretty cold, especially at night. The water could be calm as a lake or wavy. That really depends on whether you encounter a big storm system. I think you'll find the weather in most of your ports to be pretty warm, though in April there's always a chance of rain.

I assume they'll be taking a more southerly route, which generally has fairly favorable weather, though it is the open ocean so unpredictability should be expected. The prevailing Atlantic trade winds could create a bit of a bumpy ride as you're heading East, if they take the most direct route which would put you directly into the prevailing winds.

If you look at any map of the triangular trade that occurred in the days of sail, the Westerly route would send the boats south from Europe - past the Canaries and the Cape Verde Islands.

The North Atlantic that time of year can still be very, very rough. But, with a more southerly route, you should not experience frigid temperatures.

I crewed on a sailboat that went from the Canaries to the Caribbean in late October, early November. At that point of the year you're far more likley to get storms that blow from off the coast of Africa - hurricane season. I brought a lot of foul weather gear and only needed to use to a couple of nights.

I might suggest that you consider getting up a few mornings at dawn, which is a spectacular time of day on the open ocean.