Are 6-hydroxyhexan-2-one and hexanoic acid isomers?
Isomers are compounds having same molecular formula, but a different structural formula. 

As they satisfy the above condition, they are isomers and they are structural isomers. Structural isomers have a few variety based on connectivity of constituent atoms. 


Chain : which has a different arrangement of carbon molecules
Position : different position of same functional group
Functional : different functional group for the same atoms
the others (tautomerism, metamerism and ring-chain isomerism)


They are functional isomers as they have different functional groups. First one has a ketone and an alcoholic group, while the second one has a carboxylic acid functional group. 

You will come to see that it is easier to recognise structural isomers compared to the rest (stereoisomerism).

When speaking about isomerism, a picture is worth a thousand words: