I could use a little advice.

[Steel General]

Welcome Aboard!

[Midgardsormr]

When making patterns, I find it is helpful to use the Offset filter. Filter > Other > Offset…

Set Undefined Areas to Wrap Around. Your pattern will be rearranged so that what had been the edge is now in the middle, allowing you to easily touch up that line.

The pattern itself doesn't look too bad, I think. You could use some irregularity in the edges, though. They're too smooth, even in comparison to your coastline, which is also pretty smooth. You might look for one of the creating random coastlines tutorials—most of them are for the Gimp, but there are Photoshop adaptations.

Finally, have you seen the Zombie Nirvana tutorials on YouTube? They're the best place to start for Photoshop mapping, in my opinion.