I've still a long way to go, but I'm glad to share a few tips I got along the way -
Plan your map and know what you're doing. I'm saying this now but I can't seem to get this much into practice lol. Unlike digital stuff, watercolour can't be much worked over again, and if you haven't planned your map you end up with a mess.
Don't rush. Unless you want shapeless blobs.
To get a good, clean wash on the sea, I use a mixture of half-paint half-water which I spread carefully around the edges of the map (fat lot of good that actually does me - I always overlap). If it's too shallow, I add another wash on top(you can get pretty cool textures by adding wettish paint on a drying wash,without spreading it everywhere). But be careful, too many washes on the sea and it's mud.
I don't yet have a large round brush, but I recognise that using one would be useful, as in maps which don't have much sea, I always use my medium round brush,which makes it oh so easier than using a flat brush.
If you're doing the linework first, I'd recommend painting the mountains either before the landmass, or painting a wash over both the landmass and the mountains, then painting over the mountains. You don't want to get mountains that have pieces of blank paper around its corners.
Also,I'll just share this little thing... To get a decent land wash, I make one rather wet yellow ochre/brown wash, depending on the opacity I want. For the yellow/ochre, I let it dry, and then either add brown and then green (I usually use Hooker's green as I love it). Brown and green over the yellow ochre will make darker landmass with a textured look, while yellow ochre and then green will keep a light, bright look.
I'll add this, which I've read in a watercolour tutorial book - if it looks right when it's wet, it's wrong.
It is very true that the dried result is very different from what you see when the thing's wet.
Cheers