Diving instructor

I have no experience as a diving instructor, but I would write it off over 3 or 5 years. You can use your own experience for this. How long does it take for that equipment to wear out? The same applies to private use. It is important to make a good estimation of how much you will use it for private purposes. You could say, for example: I work 5 out of 7 days with that equipment: 5/7 gives 0.71% for work. If you have another (old) set that you use for personal use, that could be 100%.A lot depends on the size and type of company you have. Use common sense and make sure you have a correct and consistent story ready in case of an inspection. Google can help with this ;-)

Clear, thx

Why can I also write off equipment such as diving gear (i.e. enter it as an expense and then write it off over ... years in dexxter? What is the advantage for me here?

Your taxable income decreases, so you pay less taxes;-). If you are starting a company (e.g. as a secondary occupation), losses can even be deducted from your income as an employee. But I would advise you to consult an accountant to discuss this.(I did this myself when I started up).