i don't rely on blue tape anymore.  "frog tape" is the best product i've used for crisp lines.  it's green and the directions say to moisten it prior to painting.  it "shrink wraps" itself to the surface yet is easily removed.  it's available at the big box stores, a little pricey but isn't everything that works... 

p.s. worked great on my wd10 fiberglass deck over-wrap too 

Thanks...that sounds promising...I will raise up the water line 1/4" and try the "frog"...the wife says i should work out all these little problems before I build hers...I didn't know I was building her one ?!