You can try using the operator "OR" to concatenate keywords in your search query, like this:

query = '"maritime heritage" OR "shipwreck" OR "shipwrecks" OR "coastal archeaology" OR "maritime archaeology" OR "maritime ecologies" OR "marine archeaology" OR "marine heritage" OR "cultural marine heritage" OR "marine culutral haritage" OR "maritime history"'

# Searching in all subreddits  
all = reddit.subreddit("all")
df = pd.DataFrame() # creating dataframe for displaying scraped data

# creating lists for storing scraped data
titles=[]
scores=[]
ids=[]

# looping over posts and scraping it
for submission in all.search(query, limit=None):
    titles.append(submission.title)
    scores.append(submission.score) #upvotes
    ids.append(submission.id)


df['Title'] = titles
df['Id'] = ids
df['Upvotes'] = scores #upvotes

print(df.shape)
df.head(10)