A) Are you sure that they are adjusted properly and/or the existing pads aren't simply worn? I ask this only because my favourite bike has inexpensive (comparatively) Tektro brakes and (soon to be replaced due to wear) stock pads. They are absolutely fine in wet and dry. My other bike not only has Weinmann brakes (truly second-rate
) but steel rims, which do not make for good braking in the wet; yet I can still stop.
B) I'll let someone else confirm, but I think that some Shimano pads are cross compatible, whilst others are not; the packaging should state which they will fit.
C) Don't buy Shimano; buy something like KoolStop (Their 'Salmon' pads are designed for the wet) or SwissStop. There's only a pound or two difference.