To access shamuti island you will need to have all the non legendary kanto caught data (gen1) + the seen data of the 3 legendary birds
if you have been to the island before and now the sailor refuses to get you there, make sure to check your caught data again. There was a bug that may have deleted some of them from your dex.
If that has happened you will need to catch these pokemon and obtain their data again
it would be nice if the Pokemon was highlighted with a different color similar to how it is visible in the solaceon daily
Iirc this used to be the case but it was removed after the celadon daily was reworked, therefore I am not sure if this belongs in the suggestions