Solo spearfishing for flatfish

I will spend the next two weekends in the south of Denmark spearfishing for flatfish. Unfortunately I can't find a divebuddy. I am from Germany where it is illegal to spearfish and I do not have many contacts in Denmark which makes it pretty hard to find somebody to dive with. I really don't want to cancel my trip because of that so I am planning to snorkel in shallow water where I can see the bottom and spot the fish without diving. I would only go on the occasional duck dive to secure a fish and will have a watch with me to check my dive time. My plan is to stay underwater for a maximum of 40 seconds. I am aware of the shallow water blackout and know that there is always a risk. Do you have any tips on how to lower the risk even more?

Best regards, Laurin

Thank you for the advice
Luckily I got in contact with somebody who wants to dive the same area and we are going together.