- What is the significance of this signature (if any)?
- Is it in any way related to the seven kabbalistic seals?
- Are there other similarly unusual signatures by other prominent talmidei chachamim?
A friend on Facebook shared this answer which I have not sourced yet.
his signature is because his father's possessions were all lost in a ship wreck. He was thankful to Hashem for giving him kaparah taking his money and not his life. He had special dishes prepared with a sign of a boat and made a seudas hodah. He adopted a similar signature for his name. It's written up in the Chida's own sefer Maagal Tov
As a point of interest for #s 2 and 3 in fact there a similarly ornate signature which does contain some of the kabbalistic seals. It belongs to Shlomo Molcho
Interestingly, the post speaks about the connection between Rabbi Aiyash and the Chida.
And this is the signature of the Ari:
Less ornate than the other two, but still, not your average signature.
In this blog post there are multiple different ornate signatures. like @none, the blogger writes that there's a legend that in the Azulai family they would make their signatures boat-shaped in memory of a miracle that happened to Rabbi Avraham Azulai, the Chida's great-grandfather, when he was saved from nearly sinking along with the ship that carried all of his possessions. However, this story, while nice, is likely not the reason for the Chida's signature, as there were several non-Azulai rabbis who also had fancy signatures. The blogger suggests that most likely the idea evolved somehow from Muslim calligraphy.
Here's the signature of Rabbi Avraham Yonah, a friend of the Chida:
In the same link it's suggested that perhaps RA"Y had a ship-shaped signature to symbolize his namesake, Yonah Hanavi.