How about butcher block instead? IKEA and the big box stores sell it for countertops, the price is decent, and you won't have as many issues with buckling, water damage, or what to do with the edge as you would with the ply. If the edges of the ply get wet, or if any seam around a sink gets wet, the plywood will "blow" and you will have to replace it all, anyway.

Either will be nice, especially with a speckle through it, the black will be more dramatic, it depends what suits you the most, I myself would go for black as I like contrast and I'm a bit of a drama queen lol

Water marks don't need to be cleaned off (unless they are permanent). It just means your stone needs to be sealed. You can get a high grade sealer/ impregnator at Granquartz or Regent. Seal your marble twice a year and that problem should go away. Buy one of the half liters or 4 oz bottles (the former should last you two seals at least). This is a good quality for marble: https://www.granquartz.com/Pages/ItemProfile_Detail.aspx?ItemProfileId=1748&ItemClassNo=92 And this one is particularly good for sealing marble, especially in the kitchen: https://www.regentstoneproducts.com/item_detail2.aspx?ItemCode=ZETASEAL

Black and yellow are both very strong colour choices and I feel you'd be sick of them before it was time to change them. Black also shows marks more than you'd think. Clear is fine, but have you considered a piece of art behind clear?There are several companies that do this, imageglass.co.nz had one with tuis in it which has black and yellow. As for size are you having a range hood? If so this might affect your choice of size.