The water ballast on the M26 S is accessed just underneath the companionway steps. The single ballast tank has a removable plug in the cabin that would allow you to put some bleach in there if you felt it necessary. There is a long bolt that you loosen to let water into the ballast tank once you launch and tighten once full. Loosen the same bolt once the boat is back on the trailer and all the ballast water drains out, eventually.
I've sailed a M26 S for about 50 miles on the Atlantic from Fire Island Inlet to NYC. It was a very benign day for the most part. Because of the relatively light weight of the boat (~3000#'s with ballast full) it tends to ride over the waves rather then through them as a heavier boat might. On a choppy day this tendency might make the boats motion a bit uncomfortable but if the waves are far apart it should not be a problem. We had lazy 3' swells that the M26 S seemed not to notice.
On this trip we did get some wind as we neared NYC and my GPS indicated a ground speed of around 6.5 knots, if I recall correctly. 6.5 knots is faster then my heavy Tartan 27' can do on a good day. The same could be said for most 27' keel sailboats made by Catalina, Hunter, O'Day or whomever.
There is a trailer sailor forum as well as MacGregor owner forum at SailboatOwners.com - Latest forum activity
if you want to trade notes with other Mac owners.
Most people who have never sailed a Mac 26 S or D tend to lump all the models together and deride them for no good reason. They are not overbuilt like my 1967 Tartan 27' but they are good at what they do. The cabin interior of the M 26 S is bigger then my T27 if you don't mind not being able to stand up. All boats are compromises and the M 26 S design did not sacrifice much in the way of sailing ability.