You can, but it is usually a poor way to charge and it is not kind to the engine.
Alternator output at idle is well below its rated figure, so the charge rate is slow — and a battery bank's own acceptance rate falls as it fills, which makes the last portion slower still. Hours of idling may return far less energy than owners expect.
Extended light-load running is the part that costs you. A diesel run for long periods with almost no load stays cooler than designed, which encourages incomplete combustion, bore glazing, carbon build-up in the exhaust and wet stacking. The engine that spends its life idling at anchor tends to smoke and foul long before an engine that is worked properly.
If the boat's energy budget needs regular charging at rest, the answer is usually a larger alternator with a proper regulator, more solar, a bigger house bank, or a generator sized to the real load — not more idling. Where you must idle, run at the manual's recommended charging speed rather than the lowest speed the engine will hold, and follow it with a period under proper load.