Sorting out verb tenses here:
- I have always thought he is/was a good guy.
- I have always thought a boat doesn't/didn't float.
- I have always thought the sun rises/rose in the east.
That first one functions like this: If the good guy state or condition is viewed by you as a permanent state, the verb is is fine. If it is not viewed by you as a permanent state, the verb was is used. For example: I always thought the sun rises in the east. Where the sun rises is permanent whereas being a good guy or not is not a permanent condition. However, it's possible that your view of the sun's rising has changed, in which case "rose" is fine because of that changed view. At the time, you thought it rose in the east.
The second one is a bit odd as boats are supposed to always float. "I have always thought that that boat doesn't or didn't float. The same idea explained above can work here. It depends on how you view the condition of that boat.
The third one: 3) I have always thought the sun rises/rose in the east. again depends on the permanence or impermanence of the sun AND **on your perception of that reality.
- I have always thought the sun rose in the east. At some point in the past, that is what I thought. That's fine.
- I have always thought the sun rises in the east. Where the fact of its rising is a permanent state or condition. That is fine also.
Please bear in mind that the present perfect is not what determines what comes after it.
You can have simple past with both of those:
I thought the sun rose in the east. I thought the sun rose in the east.
One use of the present perfect that learners have difficulty with and often forget is precisely that it is used to say: This occurred in the past and I am not saying when because it is irrelevent to my idea in this sentence.* It's just past. We have no idea and don't care about when. This usage here is not about "starting in the past and continuing in the present." This usage is that the idea is situated in the past and is true (careful: true to the speaker, not absolutely) at the time of speaking.
Reminder of uses of present prefect:
The present perfect tense is used to express actions happening in an incomplete period of time. An incomplete period is one that includes the present moment, that is, the time of speaking. This period can be implied or expressed with a time expression (today, this week, this year, ever, never etc.):