Anyone using a map plotter on a boat immediately understands the whole
concept behind this question.
You make an excellent point that for people with specific domain knowledge, the question is perhaps easier to understand.
But your assumption that "stupid" people here, who do not know about the subject, closed it out of ignorance is wrong, in my opinion.
Domain knowledge does make it easy to understand a question and answer it. But the reason this specific question was closed has nothing to do with that aspect. There are multiple issues with this question:
Lack of research
The professionals who contribute an answer here are sharing their valuable experience and time for free. It is in everyone's best interest to make their job easier. It is lazy on the part of a questioner to post a question here without first highlighting how they have tried to resolve the issues. If they don't, it means more work for those who try to answer the question and this will discourage the professionals from contributing their valuable time here.
Allowing such questions can encourage other new members to imitate that style and reduce the overall quality of questions here, and thus StackExchange communities in general tend to demand more effort from the questioner. (More details here: How much research effort is expected of Stack Overflow users? - the accepted answer to that question is of a harsher tone than necessary, but nevertheless quite relevant).
Lack of clarity / details
How can I use Open Sea Maps on a Mac?
As someone who wanted to try and answer OP's question, my first obvious thought was, "Is this a newbie mac user asking how to install a particular software on a mac? Does OP want a step-by-step guide to do so? Or has OP already tried installing it and is getting some error? Online search doesn't show a mac app for this, but only a website. So what problem is OP having in running a website on a mac?" ... and so on.
I want to make waypoints on sea marks, lighthouses, islets and similar
Again, the question that comes to mind is - "What specific difficulty is OP facing in doing that? Has OP searched online or read the user manuals?
and export to the plotter I use in my boat.
Ok, what is the issue here - does he not know how to export the data or is he having some hardware issue?
(Note that all this could have been easier to deduct and understand if OP had clarified in his question the research he had done and / or the steps he had tried so far).
(Please refer: What topics can I ask about here?)
In its current form, an argument can also be made that the question is off-topic for AD (in my opinion). Just because a software is available for mac, doesn't mean this is the best place to ask a question about that software. While one can run MS Office on a macOS, it doesn't mean that questions particular to MS Office (or any other non-Apple software) would also be treated at par to more on-topic questions. The general exceptions are:
The question is related to how the software and / or its options are different on macOS (e.g. "On Windows I did this on xy software this way, but in mac this option isn't present on xy software - how do I do it on a mac?").
The question is about using the software with some other mac software or service (e.g. "I would like to save my Microsoft Word document on iCloud. Is this possible?")
The question is about particular issues with the software on macOS (e.g. "I have Microsoft Word installed. But all word documents
*.docx only open in Apple Pages when I double-click it. How can I open it on Microsoft Word?).
(These aren't hard and inflexible rules, but broad suggested guidelines - the AD community here is more accommodating than other StackExchange communities that are stricter).
I'd also like to emphasize another important point - communicating by text posts online is difficult because the lack of non-verbal cues makes it easy to misinterpret or misunderstand something.
When people ask for clarifications in comments, or mods close a question, they aren't being hostile - they are seeking clarification. Please see it from that perspective instead of thinking people are being "stupid" or "difficult". Sometimes people may indeed misunderstand something obvious. But, be kind - please remember that everyone here is contributing their time and experience for free.