Access S3 bucket from other account from EC2 instance without bucket policy

Yes, you can do that by IAM policies and resource-based ACLs or If that s3 bucket is public then also bucket policy would not be required. When I say public, depending on your use case, whether it's granted public access for read or write or both.

Please refer IAM policies and resource-based ACLs at cross account s3 bucket access.

But if either of these two options is not appropriate/feasible for use case, then answer would be no, you'll have to add bucket policy in Account B to allow the Account A EC2 Instance profile role. Also note that, ACLs are discouraged over bucket policy but if your use case has a requirement not to use bucket policy at all, then ACL is an option.

Also, note that cross account s3 access is only possible with SSE-S3 or SSE-KMS CMK, if bucket in account B is encrypted using AWS managed key SSE-KMS(aws/s3) then cross account s3 bucket access is not possible. Reason being AWS managed key(aws/s3) is unique for each account and can't be used by other account to encrypt objects.

With SSE-KMS CMK encrypted bucket, you'll have to add Account A EC2 Instance profile role in Account B KMS key policy too.

Hope you find this information helpful.

Comment here if you have additional questions, happy to help.

Abhishek