I'm going to guess you're using the paint bucket to fill your shape? That tool is odd to me. I pretty much never use it. The Fill & Stroke panel is the only way I colour objects. Is your land shape a closed object? If not, is there a way you can easily make it closed?

If you must use the bucket fill, I think there's a setting somewhere that you can tweak on how it fills, though my understanding is that the more you zoom in, the more accurate the fill will be. But that - as you've noticed! - is a bit of a nightmare with a complex shape.

As for hatching, there are several patterns available, again, in the Fill & Stroke panel, and these can be resized to suit. If none of them suit, you can make your own pattern (Create your pattern, then convert it to a pattern via the Object menu / Pattern / Objects to pattern.)

I love Inkscape for mapping, though I usually use it in combination with other tools, but for things like tapered rivers and labelling, it can't be beat (in my humble opinion!)