Use the in keyword:
something like this:
print('Sea'in 'Seahorses live in the open sea.')
If you don't want it to be case sensitive. convert all the sting to lower or upper
something like this:
string1 = 'allow'
string2 = 'Seahorses live in the open sea Allow.'
print(string1.lower() in string2.lower())
or you may use the find method like this:
string1 = 'Allow'
string2 = 'Seahorses live in the open sea Allow.'
if string2.find(string1) !=-1 :
    print('yes')
If you want to match the exact word:
string1 = 'Seah'
string2 = 'Seahorses live in the open sea Allow.'
a = sum([1 for x in string2.split(' ') if x == string1])
if a > 0:
    print('Yes')
else:
    print('No')
update
you need to ignore all the punctuation so use this.
def find(string1, string2):
    lst = string2.split(' ')
    puctuation = [',', '.', '!']
    lst2 = []
    
    for x in lst:
        for y in puctuation:
            if y in x[-1]:
                lst2.append(x.replace(y, ''))
        lst2.append(x)
    
    lst2.pop(-1)    
    a = sum([1 for x in lst2 if x.lower() == string1.lower()])
    if a > 0:
        print('Yes')
    
    else:
        print('No')
find('sea', 'Seahorses live in the open sea. .hello!')