I agree with Tika, the BlueLab pH pen is great.
It might be a bit expensive but it's accurate and will last years and years when looked after.
It's fully waterproof (I haven't tested this), has a hold feature, auto off and shows you water temp which you can change between C/F.
It shows you on the display when it should be recalibrated (approximately every 30 days, pretty sure it goes by days, not because it knows it's no longer accurate), I've checked the readings using the 4.0 and 7.0 solution before recalibration a few times, twice it was still spot on and once it was up by .1