For things that require personal fittings, you want to buy these from your LDS. Masks fall in this category. I'll fitting masks can easily ruin a dive, or make OW training a lot more difficult than it needs to be. Faces/heads are so different and masks comes in all shapes and sizes. If don't get it properly fitted, you have a high chance of getting a mask that is not optimal. Worse case, you get a mask that doesn't work at all for you.
For things that don't require personal fittings, then you can safely buy off the internet. Compasses and snorkels fall in this area.
Everything else kinda falls within the two extremes. If you're can safely buy off the rack, and are confident in matching the manufacturer's size charts, then you can buy boots and gloves online too. But if have to try on a lot of shoes in different sizes, then buying boots online would be risky.
As for fins, it's definitely cheaper to buy online. However, you have to make sure that they fit your boot. One of the most recommended fins on Scubaboard is notorious for being difficult to match with boots. Kidspot put a good site about all the
jet fins and boot sizes.
My preferences:
Masks - brand doesn't matter, whatever fits your face the best. Ideally something low-profile. Tusa Imprex, Oceanic Shadow, and Cressi Focus have worked well for me.
Snorkel - I like the simple, cheap, no frills snorkel like the
Zeagle Flex Purge. Dry snorkels are too bulky for me. The foldable snorkel is like the
Apollo Flex Snorkel is nice. But since it doesn't have a flexible bottom section, it is a bother if you wear it 100% of the time.
Fins - There is much discussion about fins here on the board. Hours of enjoyable reading, guaranteed. My preference is for paddle type fins.
Mares Avanti Quattros, Cressi Master Frogs, and
Scubapro Jet Fins have all worked well for me. My buddy retired her Jet Fins when she tried the
Twin Speeds.
Gloves - I had asked about
cold water glove recommendations, and that should be a good read. Except for below 48 deg, the
Henderson 7mm Hyperstretch gloves have been good to me.
Compass - I'm surprised Wallin's requires you to buy one. I'll let you borrow a wrist mounted one I have. PM me if you're interested. Otherwise, if you need to buy one, then the
Suunto SK7 is slick.
Since you're diving in Monterey, you might want to read this too:
Monterey divers - what gear do you use?
Have a great time in OW.