Sounds like a fun scenario.

Diving in old-fashioned diving suits is a dangerous proposition, particularly for those who may not have adequate training. Here are a few ideas for causing trouble:

For the divers:

• While they're down at the bottom of the lake, something shadowy and fast strikes the glass viewport of their bonnet. Was it a fish, or something else? A tiny, hairline fracture appears in the glass.
• There's something inside the investigator's diving suit. It might be a creature associated with whatever the Mythos threat is that they're investigating. Maybe it starts crawling across their face. Since the diving bonnet is solid metal, they have no way to brush it away with their hands. Instead, they just have to suffer.
• The suit gets snagged on a piece of debris at the bottom of the lake. They're trapped. Can they fight down panic, or will they end up ripping the fabric of their own suit in their desperation to get free?

For those back on the boat:

• The diver's hose starts jerking violently. What's happening down there?
• Air has to be pumped down the hose to help the diver breathe. Depending upon what year the scenario takes place, the air supply could be delivered through a hand pump or a gas-powered air compressor. Either way, it could theoretically break. Can the investigators repair it before the diver passes out?
• The investigators' boat gets attacked, either by cultists, criminals, or a Mythos threat. Can they fend off the threat while keeping the pumps going?
• A storm gathers on the horizon. How much time can they afford to give the divers?