antifreeze

I have a container of Seafit -50 Marine/RV antifreeze that says "this product is intended for winterizing and is not an engine coolent." Does this mean that it will not work well in my watercooling system or is it ok to use? I plan on using about 10% of it to 90% distilled water, which is about 5oz of antifreeze.

It may be extremally agressive fluid - what basically means that it may damage your block or pump. Tubing is rather resistant against such things.

Ok well i also have some Coolmax All Season Summer Coolant with anti rust ingredient. Would this be a better choice?

In my opinion - yes.
I don't know this brand, but I can advice to only pick the top quality products. There's a difference in prices (avarage quality <> top quality), but you don't buy it often. Just get the best possible.