Go on amazon or some such and look for waterproof notebooks and space pens. Together you can literally write under water. (I got them after one of those super wet games where my paper was disintegrating--not that expensive.)
I don't go for expensive stuff for rain. I use my match card as far and long as i can write on it. I make extra effort to keep it dry by protecting it from the rain (eg, use wallet and back pocket and cover it with body when writing on it). I get a new dry card at half time. Usually the last 10 minutes or so in each half may be troublesome and a wet match card if heavy rain. I rely on my memory (and my AR's for subs) in those rare times. Never missed a complete record of the game.
When the weather is really, really bad there isn't a lot you can do. The all weather notebook that Ross linked is excellent, but on one especially bad overseas thunderstorm it turned into what can only be described as a pulpy mush. But at the same time the pitches turned to lakes, even the 3Gs, changing rooms became swimming pools, not much you can do when that happens. Ideally you need to try to memorise the score and any caution numbers, anything else like times of cautions and names of players you can sort out afterwards.
My FA observer notebook, which is usually water repellent, went the same way last week ..!