Sail your Waverider and clear out every monster encampments you can find. When you finally found hilichurl encampments, you can attack them with Waverider armament or just defeat them directly with your character.
I found some guide from Paimon in hoyolab forum, here’s the details:
Waverider Armaments
The Waverider is equipped with two types of armaments — the Swiftstrike Cannon that shoots Swiftstrike Cannon and Heavy Cannon. The Waverider’s two armaments are capable of dealing considerable damage to light floating towers, but cannot damage heavy floating towers.
You can use the Waverider to destroy light floating towers and defeat enemies by sending them plummeting into the sea.
Destroy Enemy Wind Barrier
You can also climbing to platform directly and defeat the enemy. As for tips, use charged attack to throw enemies to water. If the enemy is in another platform, use archer charged attack to knock them back and throw the enemy to water.
Some enemy encampments may be shielded by a Violent Wind Barrier. Travelers will need to use the Heavy Cannon or Explosive Barrels to destroy it.
Collect pity pearls to greatly decrease the cooldown of your Heavy Cannon and bring the barriers down faster.
Beware of Whirpool!
There may be Whirlpools near the sea monster encampments. Please avoid the Whirlpools during the challenge.
If caught by the Whirlpool, the Waverider will start taking continuous damage and Travelers’ Stamina will be rapidly consumed. If the Waverider is destroyed, Travelers will automatically be ejected from the Waverider.
After defeating all enemy and reinforcements, you will obtain currency called Shiny Floatsam. By collecting this currency up to certain numbers, you can get following rewards from challenge quest and buy items from event shop.
Challenge Rewards
| Requirement | Rewards | ||
| Collect 200 Shiny Flotsam |  30 Primogems |  20,000 Mora |  1x Waverider Repair Toolbox |
| Collect 1,000 Shiny Flotsam | 30 Primogems | 20,000 Mora | – |
| Collect 2,000 Shiny Flotsam | 30 Primogems | 20,000 Mora | – |
| Defeat 30 enemies while boarding a Floating Tower |  2x Hero’s Wit | 20,000 Mora | – |
| Defeat 60 enemies while boarding a Floating Tower | 2x Hero’s Wit | 20,000 Mora | – |
| Use the Waverider’s Heavy Cannon or explosive barrels to destroy 5 Violent Wind Barriers | 2x Hero’s Wit | 20,000 Mora | – |
Shiny Floatsam Event Shop
| Item Name | Price | Stock |
 Crown of Insight | 500 | 1 |
| Northlander Billet Trove | 500 | 1 |
 Blueprint: “A Guide in the Summer Woods“ | 100 | 1 |
 Blueprint: “A Messenger in the Summer Woods“ | 100 | 1 |
 Blueprint: “Pure Gorgeous Summer“ | 200 | 1 |
 Philosophies of Freedom | 50 | 2 |
 Philosophies of Resistance | 50 | 2 |
 Philosophies of Ballad | 50 | 2 |
 Philosophies of Prosperity | 50 | 2 |
 Philosophies of Diligence | 50 | 2 |
 Philosophies of Gold | 50 | 2 |
