I'm not going to send you advice or suggestions but simply tell what I would do in the same situation.

My past choices led me to an interesting job as a self-employed consultant, covering a senior position for a big company, having a good income, good colleagues... in a few words, I'm very happy with it.

> Now, I have what looks like an offer: it's a senior position with a big company and good money. But it's boring boring BORING work in a HIDEOUS office environment

My reply is quite simple: if you're planning to marry, buy a home and have kids, it's better to look for a secure employment, otherwise consider the following priorities (most important first):
1: you work to earn your living, so it's better to look for a good paycheck;
2: you spend at least 8hrs/day working, plus commuting time; if possible, spend them in a good environment, doing a job that you like;

Point 2 involves:
2a) doing something that helps our professional and personal growth;
2b) having a good environment (good colleagues, healthy offices and so on)

Job opportunities may offer some or all of these conditions; it's up to each person to consider what's more important.
The other opportunity is to create our own job (as a consultant, for example), maybe starting from our favourite hobby and go on as self-employed people.

> "This is a good job. Don't throw away security when it's handed to you on a plate."

Nowadays, nothing (except -maybe- working for a State Agency) is really secure.

> This is the alternative. If I find a job as a deckhand [...]

Why not?

As a temporary solution, you can get the first random job that gives you some money and then go on searching what you really want.

AP.

P.S.
Pleasepleaseplease find a good job!... there are so many frustrated and unhappy people in the world, we don't need one more... :)