Wally
I have used SW Emerald Urethane, without modifications. I liked it a lot, although it has been 10-12 months since I used. I don't recall any specifics, but application and finish were both as good as I can do with a brush (and by my on own admission I can paint - haha).
General comment - Paint formulations have a LOT of ingredients (most at concentrations far less than 1%); the more expensive paints, like Emerald, have way more than the cheaper paints. Most of these relate to how the paints 'handles' (brushes or sprays), how it flows out (related to but not exclusively to open time), and of course what the final properties are (aesthetic, like gloss level, and performance, like scrubbability).
I typically avoid messing with the chemistry of a formulated product (like paints, stains), except to dilute at times. Often adding as little as 5% water (and stirring well) helps flow and brush marks, although I do not know why you'd want to increase open time. When I do this, I only add water to the transferred amount (what I expect to use), not to the source gallon or 5 gallon bucket; that way if I add too much water I can go back and add more undiluted paint. Start adding in very small amounts, it takes less than you think to modify the way the paint handles!
Are you experiencing an issue with brush marks in this product, or are you trying to prevent an issue you have seen in the past? I do not recall having a brush marks issue with Emerald, quite the opposite really - but that is a fuzzy recollection so don't quote me on that. SW has loads of people to help with technical issues and they can make recommendations. Your local store manager can be a great first resource (one of the reasons to use a 'paint store' rather than just a retailer that sells paint).