How would this hold up for saltwater aquariums. Notably, used in the sump within the cabinet which can be a humid environment ?

The problem with blue lab is the led dies. I have used 4 of these and after a year or two is a bar will go out and you cannot read the display any more. Do not buy!

can the probes stay completely submerged inside the reservoir at all times?

I've had mines for about 4 months now and it's always been in my reservoir 24-7. No problem so far.

Are the probes waterproof? Can they be used underwater?

Yes the probes CAN be submerged and are designed to stay in your liquid for extended periods (should be cleaned once a month)Youtube video from Bluelabs.com shows the EC/TDS/ppm probe submerged and this link from the manufacturer clearly states that you can completely submerged the PH probe:http://www.getbluelab.com/site/bluelabassist/files//Care%20and%20Use%20of%20Bluelab%20Soil%20pH%20Probes%20Aug09.pdf

Manufacturer	Bluelab
Item model number	BLU8001
Product Dimensions	9.14 x 21.59 x 13.21 cm; 408.23 g
ASIN	B00KCT15Q2

Bluelab MONGUACON ガーディアンモニター接続 pH、温度、導電率測定、簡単なキャリブレーションとデータロギング (コネクトスティックは付属しません)

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