Using the head light from the merchant would really only work if you have a second player at teh front of your boat, otherwise you just light up the sail and make deatils of what is in front of hte boat harder to see.

Just a little tid bit of useful trivia, the entire area out to a max of about 500 meters from the player gets generated and marked as "explored". Which can go a fair bit beyond what shows up in your view.

After the Mistlands gets added if you are not averse to using mods and wish to keep your current world I can recommend the "Upgrade Worlds" mod
https://valheim.thunderstore.io/package/JereKuusela/Upgrade_World/
it resets the map, excluding areas you define in the mod's config file, to an "unexplored" status so when you go back to that place new content can be added. (Oh, and the server devcommands mod is only required if you have a server)

Also handy is the Map Coordinate display to give you the locations to put into the config file I mentioned above.
https://www.nexusmods.com/valheim/mods/907

I've used both and they have worked exactly as descripped. After I reset my world except for my main base and a Plains Island I use for farming I re-explored a mountain top I cleared before onions were added and found onion seeds, and all the mine pits were gone too.

remember to make a backup of your world before using.