if you guys want some good all around tweak mods check out
www . nexusmods . com/witcher3/mods/73/?

remove the spaces.

Some of the improved stats are as follows;
bigger inventory weight set at 750
stamina set at 500, air set at 400, swimming stamina set at 400
toxicity set at 300
Faster Vitality Regen outside of combat
More damage for spell powers
more vitality, stamina, and panic for your horse Roach
stat improvements to armor, boots, gloves, mutagens, trophies
Geralt runs faster easier to catch animals now
Animals/Monsters now spawn 1-2 more of each and massively increased chance of getting more items from monsters
Vendors/Merchants now have 15x more money (they can buy more items from you now)
Vendors/Merchants now give you 90% of the value of the items you are selling compared to 10%
Armorers/Blacksmith repair costs for armor/weapons is reduced down by 90%
Longer potion durations increased to 31%
More skillpoints per levelup 4 to 5 each level compared to 1 with vanilla game
Reduced fall damage by automatically rolling after a jump

Hmmm a couple of those do look good.... inventory weight and merchants have more gold, to be specific. :-)

The game is easy enough already even on Death March, I don't see the point to these.

Yeh.. this would make it way too easy. It's like uber easy difficulty setting mod.

I wouldn't mind a couple of the things in the list but only as a second or perhaps third playthrough mod.. not touch the game with any gameplay altering mods for the first playthrough even if I think they need fixing because of annoying things..

I definitely endorse this mod. I am using it on my second play through and it is quite fun. Plus you can always lower some of the stats if you want less stamina etc. I'm using his full suite with the startup, patch, xml and increased stats for enemies and it makes for a great little mod. He is also very good at maintaining his mods, no keeping old patches or waiting a week for the new version.

Is it easy to turn on/off features of this mod?

Some of the changes look interesting, but the game is already easy enough on death march and a lot of these would make it ridiculously so.

Is there any mod, that increase swimming speed on boat?

Why do you guys need a mod? Just drop the stuff off in any building or street corner. Nothing will happen to it.