So if you're for example on islands world mod - which is also suggested by Dredgepack - a seed without oil on the starter island is in big trouble? Seems not great.

Even with some conveniently nearby offshore oil, you first need to research oil rigs. This takes blue science, therefore red chips, so plastic, and petrochem, which means you can't get off the ground with just underwater crude wells. It requires finding land-based oil on some other island, hopefully not too far away because you've got to secure / defend it with pre-blue-tech combat options. Not a great feel.

Adjacent things that seem slightly off:

1/ Oil rigs craft from materials accessible way before blue science, feels like after blue science maybe the recipe should involve red chips or electric engines (or big electric engines in the AAI version) or something

2/ I can just start flotation panels at red/green science with trees before I have plastic ... to mine the underwater versions of ores almost certainly on the starting island ... but still no way to get underwater oil, feels bad

You can utilize refined concrete to build out to relatively proximate deposits, as I make mention of when talking about water-surface fluid resources. This is the intended method if you're doing regular world generation with just Dredgeworks.

You raise a good point about the oil rig research, though; I'll probably add an element to Dredgepack to enable Cargo Ships' oil rigs without blue science by default (something supported with a Cargo Ships mod setting already). I usually tick that setting in my own saves, so I missed out on that adjustment propagating there.

Dredgepack is now updated with the change to aforementioned Cargo Ships mod setting's default value.