hello kasper105 - sorry for the late reply, I've been away from this site for a bit. By now you probably fixed this, but could be one of these...

1: no selection for stroking
2: foreground colour (that will be used for stroke) is the same as your water/background you're stroking on top of
3: the layer you are on to stroke could have a layer blend that is bringing it up transparent - eg. if you stroke black on a layer that is set as Colour Burn, Lighten, Screen, Colour dodge, Linear Dodge, Lighter colour, overlay, Softlight, Hard mix, Difference, etc..you may not see anything. for this step use Normal or Multiply and on a new layer and change Opacity of the layer
4: You're working on a layer under other layers that will cover what you're trying to do. Make sure it's above it. You can click-drag layers above each other.

hope that helps.

thanks everyone for your responses, you're all very welcome - happy to be of assistance. Happy mapping!