This is often caused by an older version of the client software that you are using. For example if you are using WinSCP to connect you will need to upgrade that. When I migrated from RHEL6 to RHEL7 I was on an older version of WinSCP/Putty and had to upgrade to the latest version as the older one did not have the ciphers required to connect to RHEL 7.
Yes, if you are using the Splunk Enterprise AMI located on the AWS marketplace (https://aws.amazon.com/marketplace/pp/B00PUXWXNE ), then SSH should be enabled by default. The Splunk Enterprise AMI is using Amazon Linux so the default user would be 'ec2-user' to connect via SSH. Here are additional details on how to connect to EC2 instances via SSH.