You can skip the addition of the seltzer water if you choose; however, it gives these red wine floats that "old-fashioned" soda fountain flair. You can also try using a sparkling red wine, since the bubbles in the club soda are what react to the ice cream, creating a foamy top. You could still make this float with a non-sparkling wine, though; you just have to be okay with little to no foam and may need to add a bit more ice cream to curb any sharp tastes from the alcohol.