Glowwater Potion is very strong, but early in the run it can be quite difficult to make good use of. Unless you have some cards which gain energy, drawing 10 doesn't actually let you do all that much. In contrast, Fresnel Lens is great to get early in the run, since you still have the entire rest of the run to get offered defensive cards which take advantage of it. I usually take Fresnel Lens here unless I have good energy gain and/or need a potion for an upcoming difficult fight.
You have to give up a lot of max hp to get the lens, but it's quite worth it. Compare 13 hp to 2 hp per block card you play in an act, and you can see that the Lens easily makes up for the health deficit once you add a few blocks. The Lens improves all block generated by a card, so cards that block multiple times like Dodge and Roll or Toric Toughness benefit multiple times from the enchantment. You can get the Fresnel Lens to enchant your defends if you remove and then re-add them with the Pael's Tooth blessing. Since cards can only have one enchantment, the Lens can be a downside if you end up wanting a different enchantment on your block cards.
Crossing a dreamscape of rocks and still waters, you emerge to face a sinking lighthouse emitting anti-light.
You can bottle up the eerily-glowing water that has pooled here or climb up the dilapidated structure to retrieve the lens.
Bottle
Procure Glowwater Potion.
Using a nearby bottle, you calmly scoop up the glowing liquid as the lighthouse peacefully sinks into the mud.
Good thing you weren't in there.
Climb
Obtain Fresnel Lens. Lose 13 Max HP.
You climb to the top of the lighthouse, unnerved by its steady sinking.
Upon reaching the lanthorn room, you start prying off the lens as the anti-light saps your vitality!
Finally, you wring it free; the lighthouse stops sinking.