The bug
Right clicking on a already ridden boat with no free passenger behaves inconsistently compared to all other ridable entities.
Boats act like you successfully started riding the entity and therefore the item in your hand is not used, while all others act like you did not successfully start riding them.
How to reproduce
Put an item with right click action like for example a fishing rod in your hand
Right click an already ridden minecart
/summon minecart ~ ~ ~ {Passengers:[{id:"bat"}]}
→ ✔ The fishing rod is used
Right click a boat with two passengers
/summon minecraft:boat ~ ~ ~ {Passengers:[{id:"pig"},{id:"pig"}]}
→ ❌ The fishing rod is not used and a hand animation plays instead
Linked issues
relates to
Attachments
Comments
Cannot reproduce in 1.16 Release Candidate 1

Confirmed in 1.16.1 - boats are now the only ridable entities which prevent items from being used.
Affects 21w05b
Can confirm in 21w06a.
Can confirm in 1.16.5 and 21w08b.
Can confirm in 21w10a. Video attached.
Can confirm in 21w13a.
Can confirm in 21w15a.
Can confirm in 21w17a.
Can confirm in 1.17.
Can confirm in 1.17.1.
Can confirm in 1.18.2.
Can confirm in 1.19.
Can confirm in 1.19.2.
Confirmed for 1.13.1.