A small gas leak would be less of a big deal to clean up, as gas evaporates much more readily than diesel. But a large leak would be worse. Probably still less cleanup cost, but much more safety risk.
From a cost perspective, unless you run a lot of hours, the higher fuel consumption of gas engines is likely to still come out cheaper. Insurance costs are probably not much different in the end, but purchase and maintenance costs are lower. However, the extra range of the same boat with diesels is nice, even if it's not cheaper to run.
As far as fuel burn at high vs low speeds, based on the numbers I've run for my boat, cruising on plane, diesels would burn about 60% of what my old tech carbed gas engines burn. Cruising at 7 kts, the diesels would burn about 40% of what the gas engines burn, as diesel efficiency scales better at low load, while gas engines lose a ton of efficiency at low load.
It also doesn't help that most marine gas engines compromise efficiency for being able to maintain durability with more simplistic control systems and in the hands of careless operators (otherwise they'd likely be higher compression, etc.)