Ideally, you want a unit which measures down to fractions of a mb. And records same, over the course of several days. With the ability to pull up measurements on spans of time for anything from a few hours, to several days.
Plus, & this is key. You want a unit which has alarms which you can set to go off when the air pressure changes by X amount, in Y span of time.
The user settable parts
being, X, & Y.
Also, you obviously want an old fashioned barometer. And for the on watch person, including when you're lounging on the hook, logs
the reading into your ship's log every hour on the hour.
And a watch with a built in barometer & barograph is a BIG help. You'll become Much
more in tune with the weather
after you've worn one for a bit.
I'd also recommend a small ship's weather station. Even if it's as simple as a mid-level Kestrel unit.
They have; temp graphs, barographs, wet & dry bulb sensors so as to monitor
humidity, & dew point, plus wind
meters (with graphing & recording), & more. For pretty reasonable prices. Although there are definitly perks to having sturdier, built in/purpose built units.
Although if you have a decent, built in, instruments suite onboard, you can often configure them to record
all of this data too.
But, bottom line: Definitely
get the watch with the built in baro/barograph.