What's the role/relevance of drugs when Arada taught Buddha?

Intoxicants in Buddhism are frowned on by every Buddhist school of thought.

However you didn't ask that, so we won't dive into those details.

After some time looking i could not find any evidence that Arada, or the Sankhya practices used any kind of drugs.

It was mostly types of meditation, the last three stages can be found in this article which attempts to spell out the entire process of the practice from beginning to end in a way anyone can access and understand.

Meditation on I-am-ness: Most of the time we mistakenly think that "who I am" is my mind and personality. However, as we gradually come to witness the subtle elements, the senses, and the mind itself, we come to see that there is a still subtler aspect, which simply declares "I am!" When it stands alone in this way, it is independent of the other manifestations.

Followed by...

Meditation with Buddhi standing alone: Still subtler is Buddhi, which is the individuated intelligence itself. It doesn't yet declare itself to be this or that identity, but is the very intelligence, which supports the ego (Ahamkara), the senses and instruments of actions (Indriyas), and the constructs of the inner objects and physical body (Tanmatras and Bhutas).

One of the final resting places of the individuated person is to know oneself as Buddhi, this most fine vehicle of consciousness (Purusha). It is still constructed of Prakriti, leaving that final discrimination or uncovering yet to be done. To know oneself at this level of Buddhi is sometimes called the level of bliss or ananda, as all of the other levels and false identities have temporarily come to rest or been transcended.

and finally ...

Purusha resting in itself: Finally Purusha, pure consciousness, rests in itself, alone, separate from all evolutes of Prakriti. The seeker on the path of Self-realization seeks even a minute or a moment of this highest glimpse of Realization, after which he or she continues to purify the remaining samskaras and karmas. (See Yoga Sutras 1.3 and 3.56)

These descriptions, as noted in the article, are designed to give the reader a general feel of the process.

To use your metaphor

You're on a ship → then the ship's gone → the water's gone → you're gone.

We can say the ship is what disappears during Meditation on I-am-ness The water during the Meditation with Buddhi standing alone

The last part of the metaphor is slightly misleading to match up with the last stage, it is not you're gone, but the ego that disappears, sometimes refereed to as Ego Death by mystics and psychedelic users. The last stage of Sankhya Purusha resting in itself could be compared to Ego death, and is a kind of pure conciousness that does not contain an I or an Ego, or any kind of self identity. This is the goal of the practitioner.

This is also the hardest thing to ever put into words. So in conclusion and summary : did the school and practitioners use psychedelics, likely no, they did not. These states can be reached in meditation. Are these states similar to some reported psychedelic experiences, yes.

However the key difference is that psychedelics are somewhat unpredictable. You could take the same substance and have two incredibly different experiences, and even experiences completely the opposite of Ego Death. Whereas the practices of the Sankhya are designed to always lead to the same place, with time and effort into the practices.

Further I think it is worth mentioning that Buddha moved on from this school, was not satisfied with the result as a goal. He even stated it lay on one extreme of the spectrum which the middle way lay between it and the other end of the spectrum.