I forgot about Riley, though I don't think they ever said that's why he left the military. That's a bit obvious. Its not entirely about the actual shield, it's about what it represents. Captain America was created as a super-soldier to fight against the Nazi's and the Red Skull. He used that shield and it became an iconic symbol--as did the mantel of Captain America--, it doesn't matter if that isn't technically the same shield, even though it kinda is considering that Cap probably jacked it from another timeline.
After Rodgers was believed to have died the government tried to recreate the serum, and by extension another Captain America, by experimenting on black soldiers. Some soldiers got captured and Bradley broke out and rescued them but was tossed in prison for 30 years and experimented on for doing the exact same thing that Rodgers did.
After Sam was given the shield and he decided to give it up and put it in a museum, with the government's encouragement, they then immediate decided to take it out and give it to some white guy. It supports Bradley's belief that they'd never let a black man be Captain America and that the government sees black people as disposable and less than because they were willing experiment on them in the first place.
Technically, the only reason why Sam became Captain America was because Bucky and Sam took it from him and it seems like Walker never told the government who had it. Also, they couldn't just take it back because they don't technically own it. We can't go off the comic's, in the show Bradley doesn't even want to look at the shield because he's disgusted my it.
The boat wasn't filler, it fits his storyline pretty well. It shows his stubbornness to hold onto tradition and the importance of legacy to him. He gave up the shield to preserve its legacy. Upon meeting Bradley, his world view changes and he suddenly believes that the shields may not be worth preserving because of what it represents and what it upholds. After his second meeting, he become pessimistic about becoming Captain America after taking the shield from Walker.
Time passes, and he gets more time to reflect on what it would mean to be a black Captain America while working on the boat with Bucky. As they repair the boat, their repairing its legacy. He decides that he can repair the legacy of the mantle and the shield by taking them up.